Reactor support. (#1565)

* Reactor support.

This implements the new WASI ABI described here:

https://github.com/WebAssembly/WASI/blob/master/design/application-abi.md

It adds APIs to `Instance` and `Linker` with support for running
WASI programs, and also simplifies the process of instantiating
WASI API modules.

This currently only includes Rust API support.

* Add comments and fix a typo in a comment.

* Fix a rustdoc warning.

* Tidy an unneeded `mut`.

* Factor out instance initialization with `NewInstance`.

This also separates instantiation from initialization in a manner
similar to https://github.com/bytecodealliance/lucet/pull/506.

* Update fuzzing oracles for the API changes.

* Remove `wasi_linker` and clarify that Commands/Reactors aren't connected to WASI.

* Move Command/Reactor semantics into the Linker.

* C API support.

* Fix fuzzer build.

* Update usage syntax from "::" to "=".

* Remove `NewInstance` and `start()`.

* Elaborate on Commands and Reactors and add a spec link.

* Add more comments.

* Fix wat syntax.

* Fix wat.

* Use the `Debug` formatter to format an anyhow::Error.

* Fix wat.

crates/c-api/include/wasmtime.h

@@ -117,6 +117,18 @@ WASM_API_EXTERN own wasmtime_error_t* wasmtime_linker_instantiate(
     own wasm_trap_t **trap
 );
 
+WASM_API_EXTERN own wasmtime_error_t* wasmtime_linker_module(
+    const wasmtime_linker_t *linker,
+    const wasm_name_t *name,
+    const wasm_module_t *module
+);
+
+WASM_API_EXTERN own wasmtime_error_t* wasmtime_linker_get_default(
+    const wasmtime_linker_t *linker,
+    const wasm_name_t *name,
+    own wasm_func_t **func
+);
+
 ///////////////////////////////////////////////////////////////////////////////
 //
 // wasmtime_caller_t extension, binding the `Caller` type in the Rust API

crates/c-api/src/linker.rs

@@ -1,8 +1,8 @@
 use crate::{bad_utf8, handle_result, wasmtime_error_t};
 use crate::{wasm_extern_t, wasm_store_t, ExternHost};
-use crate::{wasm_instance_t, wasm_module_t, wasm_name_t, wasm_trap_t};
+use crate::{wasm_func_t, wasm_instance_t, wasm_module_t, wasm_name_t, wasm_trap_t};
 use std::str;
-use wasmtime::{Extern, Linker};
+use wasmtime::{Extern, HostRef, Linker};
 
 #[repr(C)]
 pub struct wasmtime_linker_t {
@@ -89,3 +89,33 @@ pub unsafe extern "C" fn wasmtime_linker_instantiate(
     let result = linker.linker.instantiate(&module.module.borrow());
     super::instance::handle_instantiate(result, instance_ptr, trap_ptr)
 }
+
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_linker_module(
+    linker: &mut wasmtime_linker_t,
+    name: &wasm_name_t,
+    module: &wasm_module_t,
+) -> Option<Box<wasmtime_error_t>> {
+    let linker = &mut linker.linker;
+    let name = match str::from_utf8(name.as_slice()) {
+        Ok(s) => s,
+        Err(_) => return bad_utf8(),
+    };
+    handle_result(linker.module(name, &module.module.borrow()), |_linker| ())
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_linker_get_default(
+    linker: &mut wasmtime_linker_t,
+    name: &wasm_name_t,
+    func: &mut *mut wasm_func_t,
+) -> Option<Box<wasmtime_error_t>> {
+    let linker = &mut linker.linker;
+    let name = match str::from_utf8(name.as_slice()) {
+        Ok(s) => s,
+        Err(_) => return bad_utf8(),
+    };
+    handle_result(linker.get_default(name), |f| {
+        *func = Box::into_raw(Box::new(HostRef::new(f).into()))
+    })
+}