Stat implementation for Swift System

[jrflat]

This PR adds a Swift Stat implementation for the C stat types and system calls.

See the pitch at https://forums.swift.org/t/pitch-stat-types-for-swift-system/81616
See the full proposal in #257

[jrflat]

Note that the implementation uses typed throws which require Swift 6.0+, so earlier Swift versions are failing.

[stephentyrone]

I vaguely remember that this may cause an issue with the glibc module if another package imports it without this define? I.e. either everyone has to import it this way or no one can (in which case, we really want the C importer to set this instead of setting it in the package). @DougGregor?

[rauhul]

I would really like native TimeSpec and TimeVal wrappers to be provided by this API and conversions to/from duration be based on them.

These type represent the granularity the system thinks in and papering over that with Duration/UTCClock.instant is silly. The system cannot represent all of these values. Swift-system is supported to provide type-safe access to the system api not high level abstractions, thats best left to Foundation.

[jrflat]

I would really like native TimeSpec and TimeVal wrappers to be provided by this API and conversions to/from duration be based on them.

The stdlib already provides initializers to convert timespec and timeval to/from Duration, so I'm not sure if it's worth it to have wrappers in these cases. I think exposing the raw timespec as we're doing here is a decent compromise to allow access to the system's granularity while later providing a more ergonomic UTCClock.Instant property.