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

tests/wast_testsuites.rs

@@ -1,5 +1,5 @@
 use std::path::Path;
-use wasmtime::{Config, Engine, HostRef, Store, Strategy};
+use wasmtime::{Config, Engine, Store, Strategy};
 use wasmtime_wast::WastContext;
 
 include!(concat!(env!("OUT_DIR"), "/wast_testsuite_tests.rs"));
@@ -15,7 +15,7 @@ fn run_wast(wast: &str, strategy: Strategy) -> anyhow::Result<()> {
         .wasm_multi_value(wast.iter().any(|s| s == "multi-value"))
         .strategy(strategy)?
         .cranelift_debug_verifier(true);
-    let store = HostRef::new(Store::new(&Engine::new(&cfg)));
+    let store = Store::new(&Engine::new(&cfg));
     let mut wast_context = WastContext::new(store);
     wast_context.register_spectest()?;
     wast_context.run_file(wast)?;