Remove the need for HostRef<Module> (#778)

* 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.

* Fix compliation of test programs harness

* Fix the python extension

* Update `CodeMemory` to be `Send + Sync`

This commit updates the `CodeMemory` type in wasmtime to be both `Send`
and `Sync` by updating the implementation of `Mmap` to not store raw
pointers. This avoids the need for an `unsafe impl` and leaves the
unsafety as it is currently.

* Fix a typo

crates/api/examples/multi.rs

@@ -54,7 +54,7 @@ fn main() -> Result<()> {
 
     // Compile.
     println!("Compiling module...");
-    let module = HostRef::new(Module::new(&store, &binary).context("Error compiling module!")?);
+    let module = Module::new(&store, &binary).context("Error compiling module!")?;
 
     // Create external print functions.
     println!("Creating callback...");