Integrate New Nimble Matchers

**Do not merge yet**. It's almost ready, but I have some compiler stacktraces when trying to run the example app. But feedback is also welcomed.

See issue #90 for context. Nimble is now a submodule under Externals/Nimble.

@modocache, you mentioned something about using Adapters for some Quick-specific integration which I forgot what for. I didn't do any specific changes there, but it seems to be working as-is right now.