[RFC] impl mch_open with libuv

ref #133

update: leaving `mch_fopen` and `fdopen` (pointed out by @aktau below) for another PR.
> Another possibility is to skip the #define -> enum transform and to just define them as Lua variables 

I think that would be fine, assuming we can use the same values on all platforms. 

> Don't have a lot of time now

No worries, thanks for the reply.