core_io vs genio vs core3

This is a copy of libstd::io with all the parts that don't work in core removed. Most importantly, it provides the Read and Write traits. This crate is (mostly) automatically generated from the rust git source. The version of the source that corresponds to your compiler version will be selected automatically by the build script.

A type safe, low level replacement for `std::io`. Supports `no_std` for embedded development, just disable cargo feature `std`. Because of limitations of `std::io::Error` type, `genio` provides `Read` and `Write` traits that allow implementors to choose their own type. This type can be better at expressing what kinds of error can happen.

The bare essentials of std::io for use in no_std. Drop-in successor to core2. core2 alternative with some additional utils.