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/wasi/src/old/snapshot_0/instantiate.rs

@@ -14,12 +14,12 @@ use wasmtime_runtime::{Imports, InstanceHandle, InstantiationError, VMFunctionBo
 
 /// Creates `wasmtime::Instance` object implementing the "wasi" interface.
 pub fn create_wasi_instance(
-    store: &wasmtime::HostRef<wasmtime::Store>,
+    store: &wasmtime::Store,
     preopened_dirs: &[(String, File)],
     argv: &[String],
     environ: &[(String, String)],
 ) -> Result<wasmtime::Instance, InstantiationError> {
-    let global_exports = store.borrow().global_exports().clone();
+    let global_exports = store.global_exports().clone();
     let wasi = instantiate_wasi(global_exports, preopened_dirs, argv, environ)?;
     let instance = wasmtime::Instance::from_handle(&store, wasi);
     Ok(instance)