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