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/instance.rs

@@ -30,7 +30,7 @@ impl Resolver for SimpleResolver {
 pub fn instantiate_in_context(
     data: &[u8],
     imports: Vec<(String, String, Extern)>,
-    mut context: Context,
+    context: Context,
     exports: Rc<RefCell<HashMap<String, Option<wasmtime_runtime::Export>>>>,
 ) -> Result<(InstanceHandle, HashSet<Context>), Error> {
     let mut contexts = HashSet::new();
@@ -70,12 +70,12 @@ pub struct Instance {
 
 impl Instance {
     pub fn new(
-        store: &HostRef<Store>,
+        store: &Store,
         module: &HostRef<Module>,
         externs: &[Extern],
     ) -> Result<Instance, Error> {
-        let context = store.borrow_mut().context().clone();
-        let exports = store.borrow_mut().global_exports().clone();
+        let context = store.context().clone();
+        let exports = store.global_exports().clone();
         let imports = module
             .borrow()
             .imports()
@@ -131,7 +131,7 @@ impl Instance {
         Some(&self.exports()[i])
     }
 
-    pub fn from_handle(store: &HostRef<Store>, instance_handle: InstanceHandle) -> Instance {
+    pub fn from_handle(store: &Store, instance_handle: InstanceHandle) -> Instance {
         let contexts = HashSet::new();
 
         let mut exports = Vec::new();
@@ -143,7 +143,7 @@ impl Instance {
                 // HACK ensure all handles, instantiated outside Store, present in
                 // the store's SignatureRegistry, e.g. WASI instances that are
                 // imported into this store using the from_handle() method.
-                let _ = store.borrow_mut().register_wasmtime_signature(signature);
+                let _ = store.register_wasmtime_signature(signature);
             }
             let extern_type = ExternType::from_wasmtime_export(&export);
             exports_types.push(ExportType::new(name, extern_type));