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/test-programs/tests/wasm_tests/runtime.rs

@@ -43,9 +43,8 @@ pub fn instantiate(data: &[u8], bin_name: &str, workspace: Option<&Path>) -> any
         .context("failed to instantiate wasi")?,
     );
 
-    let module = HostRef::new(Module::new(&store, &data).context("failed to create wasm module")?);
+    let module = Module::new(&store, &data).context("failed to create wasm module")?;
     let imports = module
-        .borrow()
         .imports()
         .iter()
         .map(|i| {