std: add RtioTimer and UvTimer impl atop rt::uv
std: add rt::io::Timer
std: minor timer cleanup based on feedback
std: make check appeasement
auto merge of #7916 : olsonjeffery/rust/newrt_timer, r=brson

My first bit of newsched IO work. Pretty simple and limited in scope.

the RtioTimer trait only has a `sleep(msecs: u64)` method, for now. Taking requests on what else ought to be here.

oh yeah: this resolves #6435