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/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)?);