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/tests/import_calling_export.rs

@@ -34,7 +34,7 @@ fn test_import_calling_export() {
 
     let store = Store::default();
     let wasm = wat::parse_str(WAT).unwrap();
-    let module = HostRef::new(Module::new(&store, &wasm).expect("failed to create module"));
+    let module = Module::new(&store, &wasm).expect("failed to create module");
 
     let callback = Rc::new(Callback {
         other: RefCell::new(None),

crates/api/tests/import_calling_export.wat

@@ -1,8 +0,0 @@
-(module
-  (type $t0 (func))
-  (import "" "imp" (func $.imp (type $t0)))
-  (func $run call $.imp)
-  (func $other)
-  (export "run" (func $run))
-  (export "other" (func $other))
-)

crates/api/tests/traps.rs

@@ -15,17 +15,16 @@ fn test_trap_return() -> Result<(), String> {
     let store = Store::default();
     let binary = parse_str(
         r#"
-                (module
-                (func $hello (import "" "hello"))
-                (func (export "run") (call $hello))
-                )
-            "#,
+            (module
+            (func $hello (import "" "hello"))
+            (func (export "run") (call $hello))
+            )
+        "#,
     )
     .map_err(|e| format!("failed to parse WebAssembly text source: {}", e))?;
 
-    let module = HostRef::new(
-        Module::new(&store, &binary).map_err(|e| format!("failed to compile module: {}", e))?,
-    );
+    let module =
+        Module::new(&store, &binary).map_err(|e| format!("failed to compile module: {}", e))?;
     let hello_type = FuncType::new(Box::new([]), Box::new([]));
     let hello_func = HostRef::new(Func::new(&store, hello_type, Rc::new(HelloCallback)));