Added basic platform tree parser with tests

This adds platform tree parser as described in [this RFC](https://gist.github.com/farcaller/b89743924b0170674738).

This PR is not ready for committing yet, I want just a basic code review. The code is quite dirty and there seem to be lots of opportunities for optimisation.

Also, this one adds native code builds and `rake test`.
No `Context.instance.dylib_name()`?