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/misc/py/src/function.rs

@@ -126,7 +126,7 @@ impl wasmtime::Callable for WrappedFn {
 }
 
 pub fn wrap_into_pyfunction(
-    store: &wasmtime::HostRef<wasmtime::Store>,
+    store: &wasmtime::Store,
     callable: &PyAny,
 ) -> PyResult<wasmtime::HostRef<wasmtime::Func>> {
     if !callable.hasattr("__annotations__")? {

crates/misc/py/src/lib.rs

@@ -44,11 +44,7 @@ impl InstantiateResultObject {
     }
 }
 
-fn find_export_in(
-    obj: &PyAny,
-    store: &wasmtime::HostRef<wasmtime::Store>,
-    name: &str,
-) -> PyResult<wasmtime::Extern> {
+fn find_export_in(obj: &PyAny, store: &wasmtime::Store, name: &str) -> PyResult<wasmtime::Extern> {
     let obj = obj.cast_as::<PyDict>()?;
 
     Ok(if let Some(item) = obj.get_item(name) {
@@ -86,7 +82,7 @@ pub fn instantiate(
     let wasm_data = buffer_source.as_bytes();
 
     let engine = wasmtime::Engine::new(&wasmtime::Config::new().wasm_multi_value(true));
-    let store = wasmtime::HostRef::new(wasmtime::Store::new(&engine));
+    let store = wasmtime::Store::new(&engine);
 
     let module = wasmtime::HostRef::new(wasmtime::Module::new(&store, wasm_data).map_err(err2py)?);
 

crates/misc/rust/macro/src/lib.rs

@@ -52,8 +52,8 @@ fn generate_load(item: &syn::ItemTrait) -> syn::Result<TokenStream> {
             use #root::anyhow::{bail, format_err};
 
             let engine = Engine::new(Config::new().wasm_multi_value(true));
-            let store = HostRef::new(Store::new(&engine));
-            let global_exports = store.borrow().global_exports().clone();
+            let store = Store::new(&engine);
+            let global_exports = store.global_exports().clone();
 
             let data = #root::wasmtime_interface_types::ModuleData::new(&bytes)?;