[RFC] impl mch_open with libuv

ref #133

update: leaving `mch_fopen` and `fdopen` (pointed out by @aktau below) for another PR.
@justinmk 
> @aktau @Hinidu did we ever find a way to import header constants?

We have something setup in `test/includes` that was used to extract constants from `sys/stat.h`.  Perhaps something similar could be done here?  Another choice would be to build some sort of lua helper (perhaps in C) to help pull out the required values.  I'm not sure what it takes to do that, but I imagine its not too difficult.  I've thought a few times of doing it myself and making some package available on luarocks--it's disappointing that it's not there already. :-(