This commit completes the reference-types proposal implementation. Notably, this adds support for * `ref.is_null`, `ref.func`, `ref.is_null` instructions * `table.get`, `table.set`, `table.grow`, `table.size` and `table.fill` instructions * `Externref` and `Funcref` types (including invocation via uint64 encoding). part of #484 Signed-off-by: Takeshi Yoneda <takeshi@tetrate.io>