Move FdTable to a common location and split off Unix behavior

[CraftSpider]

I'm open to instead keeping all the functions on the base trait, but this seemed like an easy way to keep them clearly only intended to be used in the unix shims.

Preparation for Windows FS support, see #4043

[cgettys-microsoft]

@sivadeilra - here's one of the PRs related to adding Miri support for Windows FS APIs, RE: #3482

[RalfJung]

I'm open to instead keeping all the functions on the base trait, but this seemed like an easy way to keep them clearly only intended to be used in the unix shims.

What are you referring to here?

[bors]

☔ The latest upstream changes (presumably #4016) made this pull request unmergeable. Please resolve the merge conflicts.

[CraftSpider]

This is now 3 commits - the first only moves the FdMap, the seconds splits out Unix functionality, the third makes the fn metadata change used by the Windows PR

[RalfJung]

@CraftSpider Thanks!
This seems to have conflicts with master now, so CI cannot run. Could you resolve those conflicts?

@rustbot author

(write @rustbot ready when this is ready for review)

[CraftSpider]

@rustbot ready

[RalfJung]

@rustbot author

[CraftSpider]

@rustbot ready

[RalfJung]

Looks good, thanks! Please squash the last 2 commits in the previous ones (but definitely leave the first commit separate).

[CraftSpider]

@rustbot ready