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/src/values.rs

@@ -197,7 +197,7 @@ impl From<RuntimeValue> for Val {
 
 pub(crate) fn into_checked_anyfunc(
     val: Val,
-    store: &HostRef<Store>,
+    store: &Store,
 ) -> wasmtime_runtime::VMCallerCheckedAnyfunc {
     match val {
         Val::AnyRef(AnyRef::Null) => wasmtime_runtime::VMCallerCheckedAnyfunc {
@@ -215,7 +215,7 @@ pub(crate) fn into_checked_anyfunc(
                 } => (*vmctx, *address, signature),
                 _ => panic!("expected function export"),
             };
-            let type_index = store.borrow_mut().register_wasmtime_signature(signature);
+            let type_index = store.register_wasmtime_signature(signature);
             wasmtime_runtime::VMCallerCheckedAnyfunc {
                 func_ptr,
                 type_index,
@@ -228,16 +228,14 @@ pub(crate) fn into_checked_anyfunc(
 
 pub(crate) fn from_checked_anyfunc(
     item: &wasmtime_runtime::VMCallerCheckedAnyfunc,
-    store: &HostRef<Store>,
+    store: &Store,
 ) -> Val {
     if item.type_index == wasmtime_runtime::VMSharedSignatureIndex::default() {
         return Val::AnyRef(AnyRef::Null);
     }
     let signature = store
-        .borrow()
         .lookup_wasmtime_signature(item.type_index)
-        .expect("signature")
-        .clone();
+        .expect("signature");
     let instance_handle = unsafe { wasmtime_runtime::InstanceHandle::from_vmctx(item.vmctx) };
     let export = wasmtime_runtime::Export::Function {
         address: item.func_ptr,