core: Refactor uv bindings

I can already see these are going to get massive. Putting them into multiple
files.
Add a way to run the test suite with the new scheduler

TESTARGS=--newrt make check-stage1-rpass

Conflicts:
	src/rt/rustrt.def.in
core: Initialize global state lazily in the Scheduler ctor

I don't want any global one-time initalization functions because
that will make embedding harder.