Remove the need for HostRef<Store> (#771)

* Remove the need for `HostRef<Store>`

This commit goes through the public API of the `wasmtime` crate and
removes the need for `HostRef<Store>`, as discussed in #708. This commit
is accompanied with a few changes:

* The `Store` type now also implements `Default`, creating a new
  `Engine` with default settings and returning that.

* The `Store` type now implements `Clone`, and is documented as being a
  "cheap clone" aka being reference counted. As before there is no
  supported way to create a deep clone of a `Store`.

* All APIs take/return `&Store` or `Store` instead of `HostRef<Store>`,
  and `HostRef<T>` is left as purely a detail of the C API.

* The `global_exports` function is tagged as `#[doc(hidden)]` for now
  while we await its removal.

* The `Store` type is not yet `Send` nor `Sync` due to the usage of
  `global_exports`, but it is intended to become so eventually.

* Touch up comments on some examples

* Run rustfmt

crates/api/examples/multi.rs

@@ -46,7 +46,7 @@ fn main() -> Result<()> {
     // Initialize.
     println!("Initializing...");
     let engine = Engine::new(Config::new().wasm_multi_value(true));
-    let store = HostRef::new(Store::new(&engine));
+    let store = Store::new(&engine);
 
     // Load binary.
     println!("Loading binary...");