Remove need for HostRef<Engine> (#762)

This commit removes the need to use `HostRef<Engine>` in the Rust API.
Usage is retained in the C API in one location, but otherwise `Engine`
can always be used directly.

This is the first step of progress on #708 for the `Engine` type.
Changes here include:

* `Engine` is now `Clone`, and is documented as being cheap. It's not
  intended that cloning an engine creates a deep copy.
* `Engine` is now both `Send` and `Sync`, and asserted to be so.
* Usage of `Engine` in APIs no longer requires or uses `HostRef`.

docs/embed-rust.md

@@ -38,7 +38,7 @@ It is time to add code to the `src/main.rs`. First, the engine and storage need
 ```rust
 use wasmtime::*;
 
-let engine = HostRef::new(Engine::default());
+let engine = Engine::default();
 let store = HostRef::new(Store::new(&engine));
 ```
 
@@ -87,7 +87,7 @@ use std::fs::read;
 use wasmtime::*;
 
 fn main() {
-    let engine = HostRef::new(Engine::default());
+    let engine = Engine::default();
     let store = HostRef::new(Store::new(&engine));
 
     let wasm = read("hello.wasm").expect("wasm file");