At least portions of `std::sys` should be public

[emberian]

Right now it is impossible to use MemoryMap with a file because AsFileDesc is in sys_common and thus private.

We should figure out which portions of sys should be public.

[emberian]

cc @aturon @xales

[Arcterus]

We need AsFileDesc to fix the uutils build as well (without resorting to hacks).

[aturon]

Yes, exposing file descriptors is very much part of the plan, though they will likely not be exposed directly via sys. More on this very soon.