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/api/examples/gcd.rs

@@ -38,11 +38,10 @@ fn main() -> anyhow::Result<()> {
     // `Module` which is attached to a `Store` cache.
     let wasm = wat::parse_str(WAT)?;
     let store = Store::default();
-    let module = HostRef::new(Module::new(&store, &wasm)?);
+    let module = Module::new(&store, &wasm)?;
 
     // Find index of the `gcd` export.
     let gcd_index = module
-        .borrow()
         .exports()
         .iter()
         .enumerate()