jni-rs

Mark `MonitorGuard` with `#[must_use]`

Have `Env::get_rust_field` return a `MutexGuard<'local>`

Add Env::load_class wrapper for LoaderContext::load_class

Add ScopeTokens + consistent JavaVM/Env::with[_top]_local_frame APIs

Output a clear panic message if Env::assert_top check fails