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/tests/import_calling_export.rs

@@ -32,8 +32,7 @@ fn test_import_calling_export() {
         }
     }
 
-    let engine = Engine::default();
-    let store = HostRef::new(Store::new(&engine));
+    let store = Store::default();
     let wasm = wat::parse_str(WAT).unwrap();
     let module = HostRef::new(Module::new(&store, &wasm).expect("failed to create module"));
 

crates/api/tests/traps.rs

@@ -12,8 +12,7 @@ fn test_trap_return() -> Result<(), String> {
         }
     }
 
-    let engine = Engine::default();
-    let store = HostRef::new(Store::new(&engine));
+    let store = Store::default();
     let binary = parse_str(
         r#"
                 (module