[RFC] impl mch_open with libuv

ref #133

update: leaving `mch_fopen` and `fdopen` (pointed out by @aktau below) for another PR.
Just to chime in again, ffiex uses the `-dM` preprocessor switch for this (I think). Look at the readme: https://github.com/umegaya/ffiex

What I made is basically (I think) a weak version of ffiex. If someone has time to experiment, I'd try replacing it.