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/trampoline/func.rs

@@ -2,7 +2,6 @@
 
 use super::create_handle::create_handle;
 use super::trap::{record_api_trap, TrapSink, API_TRAP_CODE};
-use crate::r#ref::HostRef;
 use crate::{Callable, FuncType, Store, Val};
 use anyhow::Result;
 use std::cmp;
@@ -234,7 +233,7 @@ fn make_trampoline(
 pub fn create_handle_with_function(
     ft: &FuncType,
     func: &Rc<dyn Callable + 'static>,
-    store: &HostRef<Store>,
+    store: &Store,
 ) -> Result<InstanceHandle> {
     let sig = ft.get_wasmtime_signature().clone();
 
@@ -270,7 +269,7 @@ pub fn create_handle_with_function(
 
     create_handle(
         module,
-        Some(store.borrow_mut()),
+        Some(store),
         finished_functions,
         Box::new(trampoline_state),
     )