Refactor and fill out wasmtime's C API (#1415)

* Refactor and improve safety of C API

This commit is intended to be a relatively large refactoring of the C
API which is targeted at improving the safety of our C API definitions.
Not all of the APIs have been updated yet but this is intended to be the
start.

The goal here is to make as many functions safe as we can, expressing
inputs/outputs as native Rust types rather than raw pointers wherever
possible. For example instead of `*const wasm_foo_t` we'd take
`&wasm_foo_t`. Instead of returning `*mut wasm_foo_t` we'd return
`Box<wasm_foo_t>`. No ABI/API changes are intended from this commit,
it's supposed to only change how we define all these functions
internally.

This commit also additionally implements a few more API bindings for
exposed vector types by unifying everything into one macro.

Finally, this commit moves many internal caches in the C API to the
`OnceCell` type which provides a safe interface for one-time
initialization.

* Split apart monolithic C API `lib.rs`

This commit splits the monolithic `src/lib.rs` in the C API crate into
lots of smaller files. The goal here is to make this a bit more readable
and digestable. Each module now contains only API bindings for a
particular type, roughly organized around the grouping in the wasm.h
header file already.

A few more extensions were added, such as filling out `*_as_*`
conversions with both const and non-const versions. Additionally many
APIs were made safer in the same style as the previous commit, generally
preferring Rust types rather than raw pointer types.

Overall no functional change is intended here, it should be mostly just
code movement and minor refactorings!

* Make a few wasi C bindings safer

Use safe Rust types where we can and touch up a few APIs here and there.

* Implement `wasm_*type_as_externtype*` APIs

This commit restructures `wasm_externtype_t` to be similar to
`wasm_extern_t` so type conversion between the `*_extern_*` variants to
the concrete variants are all simple casts. (checked in the case of
general to concrete, of course).

* Consistently imlpement host info functions in the API

This commit adds a small macro crate which is then used to consistently
define the various host-info-related functions in the C API. The goal
here is to try to mirror what the `wasm.h` header provides to provide a
full implementation of the header.

crates/c-api/src/instance.rs

@@ -0,0 +1,111 @@
+use crate::{wasm_extern_t, wasm_extern_vec_t, wasm_module_t, wasm_trap_t};
+use crate::{wasm_store_t, ExternHost};
+use anyhow::Result;
+use std::cell::RefCell;
+use std::ptr;
+use wasmtime::{Extern, HostRef, Instance, Store, Trap};
+
+#[repr(C)]
+#[derive(Clone)]
+pub struct wasm_instance_t {
+    pub(crate) instance: HostRef<Instance>,
+    exports_cache: RefCell<Option<Vec<ExternHost>>>,
+}
+
+wasmtime_c_api_macros::declare_ref!(wasm_instance_t);
+
+impl wasm_instance_t {
+    fn new(instance: Instance) -> wasm_instance_t {
+        wasm_instance_t {
+            instance: HostRef::new(instance),
+            exports_cache: RefCell::new(None),
+        }
+    }
+
+    fn anyref(&self) -> wasmtime::AnyRef {
+        self.instance.anyref()
+    }
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn wasm_instance_new(
+    store: &wasm_store_t,
+    module: &wasm_module_t,
+    imports: *const Box<wasm_extern_t>,
+    result: Option<&mut *mut wasm_trap_t>,
+) -> Option<Box<wasm_instance_t>> {
+    let mut externs: Vec<Extern> = Vec::with_capacity((*module).imports.len());
+    for i in 0..(*module).imports.len() {
+        let import = &*imports.add(i);
+        externs.push(match &import.which {
+            ExternHost::Func(e) => Extern::Func(e.borrow().clone()),
+            ExternHost::Table(e) => Extern::Table(e.borrow().clone()),
+            ExternHost::Global(e) => Extern::Global(e.borrow().clone()),
+            ExternHost::Memory(e) => Extern::Memory(e.borrow().clone()),
+        });
+    }
+    let store = &(*store).store.borrow();
+    let module = &(*module).module.borrow();
+    if !Store::same(&store, module.store()) {
+        if let Some(result) = result {
+            let trap = Trap::new("wasm_store_t must match store in wasm_module_t");
+            let trap = Box::new(wasm_trap_t {
+                trap: HostRef::new(trap),
+            });
+            *result = Box::into_raw(trap);
+        }
+        return None;
+    }
+    handle_instantiate(Instance::new(module, &externs), result)
+}
+
+pub fn handle_instantiate(
+    instance: Result<Instance>,
+    result: Option<&mut *mut wasm_trap_t>,
+) -> Option<Box<wasm_instance_t>> {
+    match instance {
+        Ok(instance) => {
+            if let Some(result) = result {
+                *result = ptr::null_mut();
+            }
+            Some(Box::new(wasm_instance_t::new(instance)))
+        }
+        Err(trap) => {
+            if let Some(result) = result {
+                let trap = match trap.downcast::<Trap>() {
+                    Ok(trap) => trap,
+                    Err(e) => Trap::new(format!("{:?}", e)),
+                };
+                let trap = Box::new(wasm_trap_t {
+                    trap: HostRef::new(trap),
+                });
+                *result = Box::into_raw(trap);
+            }
+            None
+        }
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn wasm_instance_exports(instance: &wasm_instance_t, out: &mut wasm_extern_vec_t) {
+    let mut cache = instance.exports_cache.borrow_mut();
+    let exports = cache.get_or_insert_with(|| {
+        let instance = &instance.instance.borrow();
+        instance
+            .exports()
+            .iter()
+            .map(|e| match e {
+                Extern::Func(f) => ExternHost::Func(HostRef::new(f.clone())),
+                Extern::Global(f) => ExternHost::Global(HostRef::new(f.clone())),
+                Extern::Memory(f) => ExternHost::Memory(HostRef::new(f.clone())),
+                Extern::Table(f) => ExternHost::Table(HostRef::new(f.clone())),
+            })
+            .collect()
+    });
+    let mut buffer = Vec::with_capacity(exports.len());
+    for e in exports {
+        let ext = Box::new(wasm_extern_t { which: e.clone() });
+        buffer.push(Some(ext));
+    }
+    out.set_buffer(buffer);
+}