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/wast/src/spectest.rs

@@ -17,7 +17,7 @@ where
 }
 
 fn wrap(
-    store: &HostRef<Store>,
+    store: &Store,
     ty: FuncType,
     callable: impl Fn(&[Val], &mut [Val]) -> Result<(), Trap> + 'static,
 ) -> Func {
@@ -26,7 +26,7 @@ fn wrap(
 
 /// Return an instance implementing the "spectest" interface used in the
 /// spec testsuite.
-pub fn instantiate_spectest(store: &HostRef<Store>) -> HashMap<&'static str, Extern> {
+pub fn instantiate_spectest(store: &Store) -> HashMap<&'static str, Extern> {
     let mut ret = HashMap::new();
 
     let ty = FuncType::new(Box::new([]), Box::new([]));