Review request

[NobodyXu]

This crate contains some unsafe code required for efficient io operations, so I'd like your review @jonhoo .
It should be quite easy to review since it only has roughly 800 lines of code.

Some of them (IoSliceExt) might even be qualified to upstream to std library.

[jonhoo]

Unfortunately, I don't have the spare cycles at the moment to do a proper review of this at the moment 😞

[NobodyXu]

Unfortunately, I don't have the spare cycles at the moment to do a proper review of this at the moment 😞

Feel free to review it at anytime, though it should not take much time to review this crate because the functions are highly decoupled with relatively few unsafe.