Remove HostRef from the wasmtime public API (#788)

* Remove `HostRef` from the `wasmtime` public API

This commit removes all remaining usages of `HostRef` in the public API
of the `wasmtime` crate. This involved a number of API decisions such
as:

* None of `Func`, `Global`, `Table`, or `Memory` are wrapped in `HostRef`
* All of `Func`, `Global`, `Table`, and `Memory` implement `Clone` now.
* Methods called `type` are renamed to `ty` to avoid typing `r#type`.
* Methods requiring mutability for external items now no longer require
  mutability. The mutable reference here is sort of a lie anyway since
  the internals are aliased by the underlying module anyway. This
  affects:
  * `Table::set`
  * `Table::grow`
  * `Memory::grow`
  * `Instance::set_signal_handler`
* The `Val::FuncRef` type is now no longer automatically coerced to
  `AnyRef`. This is technically a breaking change which is pretty bad,
  but I'm hoping that we can live with this interim state while we sort
  out the `AnyRef` story in general.
* The implementation of the C API was refactored and updated in a few
  locations to account for these changes:
  * Accessing the exports of an instance are now cached to ensure we
    always hand out the same `HostRef` values.
  * `wasm_*_t` for external values no longer have internal cache,
    instead they all wrap `wasm_external_t` and have an unchecked
    accessor for the underlying variant (since the type is proof that
    it's there). This makes casting back and forth much more trivial.

This is all related to #708 and while there's still more work to be done
in terms of documentation, this is the major bulk of the rest of the
implementation work on #708 I believe.

* More API updates

* Run rustfmt

* Fix a doc test

* More test updates

src/commands/run.rs

@@ -11,7 +11,7 @@ use std::{
 };
 use structopt::{clap::AppSettings, StructOpt};
 use wasi_common::preopen_dir;
-use wasmtime::{Config, Engine, HostRef, Instance, Module, Store};
+use wasmtime::{Config, Engine, Instance, Module, Store};
 use wasmtime_environ::cache_init;
 use wasmtime_interface_types::ModuleData;
 use wasmtime_wasi::{
@@ -148,7 +148,7 @@ impl RunCommand {
         let preopen_dirs = self.compute_preopen_dirs()?;
         let argv = self.compute_argv();
 
-        let wasi_unstable = HostRef::new(if self.enable_wasi_c {
+        let wasi_unstable = if self.enable_wasi_c {
             #[cfg(feature = "wasi-c")]
             {
                 let global_exports = store.global_exports().clone();
@@ -161,14 +161,10 @@ impl RunCommand {
             }
         } else {
             create_wasi_instance_snapshot_0(&store, &preopen_dirs, &argv, &self.vars)?
-        });
+        };
 
-        let wasi_snapshot_preview1 = HostRef::new(create_wasi_instance(
-            &store,
-            &preopen_dirs,
-            &argv,
-            &self.vars,
-        )?);
+        let wasi_snapshot_preview1 =
+            create_wasi_instance(&store, &preopen_dirs, &argv, &self.vars)?;
 
         module_registry.insert("wasi_unstable".to_owned(), wasi_unstable);
         module_registry.insert("wasi_snapshot_preview1".to_owned(), wasi_snapshot_preview1);
@@ -232,9 +228,9 @@ impl RunCommand {
 
     fn instantiate_module(
         store: &Store,
-        module_registry: &HashMap<String, HostRef<Instance>>,
+        module_registry: &HashMap<String, Instance>,
         path: &Path,
-    ) -> Result<(HostRef<Instance>, Module, Vec<u8>)> {
+    ) -> Result<(Instance, Module, Vec<u8>)> {
         // Read the wasm module binary either as `*.wat` or a raw binary
         let data = wat::parse_file(path)?;
 
@@ -248,7 +244,7 @@ impl RunCommand {
                 let module_name = i.module();
                 if let Some(instance) = module_registry.get(module_name) {
                     let field_name = i.name();
-                    if let Some(export) = instance.borrow().find_export_by_name(field_name) {
+                    if let Some(export) = instance.find_export_by_name(field_name) {
                         Ok(export.clone())
                     } else {
                         bail!(
@@ -263,10 +259,8 @@ impl RunCommand {
             })
             .collect::<Result<Vec<_>, _>>()?;
 
-        let instance = HostRef::new(
-            Instance::new(store, &module, &imports)
-                .context(format!("failed to instantiate {:?}", path))?,
-        );
+        let instance = Instance::new(store, &module, &imports)
+            .context(format!("failed to instantiate {:?}", path))?;
 
         Ok((instance, module, data))
     }
@@ -274,7 +268,7 @@ impl RunCommand {
     fn handle_module(
         &self,
         store: &Store,
-        module_registry: &HashMap<String, HostRef<Instance>>,
+        module_registry: &HashMap<String, Instance>,
     ) -> Result<()> {
         let (instance, module, data) =
             Self::instantiate_module(store, module_registry, &self.module)?;
@@ -301,16 +295,11 @@ impl RunCommand {
         Ok(())
     }
 
-    fn invoke_export(
-        &self,
-        instance: HostRef<Instance>,
-        data: &ModuleData,
-        name: &str,
-    ) -> Result<()> {
+    fn invoke_export(&self, instance: Instance, data: &ModuleData, name: &str) -> Result<()> {
         use wasm_webidl_bindings::ast;
         use wasmtime_interface_types::Value;
 
-        let mut handle = instance.borrow().handle().clone();
+        let mut handle = instance.handle().clone();
 
         // Use the binding information in `ModuleData` to figure out what arguments
         // need to be passed to the function that we're invoking. Currently we take