[RFC] os: implement VimL libcall with {mch,os}_libcall

The old mch_libcall was removed from neovim. This is a partial
reimplementation on top of libuv. It doesn't catch exceptions (windows) nor
signals (unix) though, so it's quite a bit more prone to crashing if the
loadable library throws an exception or crashes. Still, it should be fine
for well-behaved libraries. Requested by @Shougo.