Remove the need for HostRef<Module>

This commit continues previous work and also #708 by removing the need
to use `HostRef<Module>` in the API of the `wasmtime` crate. The API
changes performed here are:

* The `Module` type is now itself internally reference counted.
* The `Module::store` function now returns the `Store` that was used to
  create a `Module`
* Documentation for `Module` and its methods have been expanded.

crates/misc/py/src/instance.rs

@@ -21,7 +21,7 @@ impl Instance {
         let py = gil.python();
         let exports = PyDict::new(py);
         let module = self.instance.borrow().module().clone();
-        for (i, e) in module.borrow().exports().iter().enumerate() {
+        for (i, e) in module.exports().iter().enumerate() {
             match e.ty() {
                 wasmtime::ExternType::Func(ft) => {
                     let mut args_types = Vec::new();

crates/misc/py/src/lib.rs

@@ -84,7 +84,7 @@ pub fn instantiate(
     let engine = wasmtime::Engine::new(&wasmtime::Config::new().wasm_multi_value(true));
     let store = wasmtime::Store::new(&engine);
 
-    let module = wasmtime::HostRef::new(wasmtime::Module::new(&store, wasm_data).map_err(err2py)?);
+    let module = wasmtime::Module::new(&store, wasm_data).map_err(err2py)?;
 
     let data = Rc::new(ModuleData::new(wasm_data).map_err(err2py)?);
 
@@ -99,7 +99,7 @@ pub fn instantiate(
     };
 
     let mut imports: Vec<wasmtime::Extern> = Vec::new();
-    for i in module.borrow().imports() {
+    for i in module.imports() {
         let module_name = i.module();
         if let Some(m) = import_obj.get_item(module_name) {
             let e = find_export_in(m, &store, i.name())?;

crates/misc/py/src/module.rs

@@ -4,5 +4,5 @@ use pyo3::prelude::*;
 
 #[pyclass]
 pub struct Module {
-    pub module: wasmtime::HostRef<wasmtime::Module>,
+    pub module: wasmtime::Module,
 }