core: Cleanup rt::context
Give core::rt and std::net their own uvll bindings

I intend to do some big refactoring and don't want to deal w/ std just now
core: Don't use printf in rtdebug!

The bots were showing a segfault that I can't reproduce. Assuming it's
varargs related so let's not use printf
core: Simplify uvll bindings and strip out currently-unused bits

No more mapping uv structs to Rust structs
core: Convert some multiline statements to single-line
core: Rename rt::io to rt::rtio

This is an internal interface. I want to use rt::io for public interfaces.
mk: If NO_REBUILD is set then don't rebuild core/std before testing

Can make turnaround of testing changes to core/std/syntax much faster.
core: Add to_bytes method to StrSlice trait
core: Add rt::io and start sketching the API
core: Begin uv file system bindings
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.