Replace libextobjc submodule with a subtree and namespace its symbols

Since libextobjc rarely changes, this saves clone/update time.

It also allows us to make modifications local to Mantle. This PR (re-)namespaces the used libextobjc functions to avoid symbol conflicts if another framework exposes the same symbols.

General-purpose instructions for updating the subtree can be found in the README of [jspahrsummers/objc-build-scripts](https://github.com/jspahrsummers/objc-build-scripts).