wipschedlines
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
wipio
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.
wipuv
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.