[DRAFT][SKETCH] Ideas for stat.h interface

[rauhul]

• Adds wrapper functions for all posix functions in darwin's stat.h header.
• Non-posix stat.h functions (<fn>_np) are out of scope of this sketch.
• fcntrl functions are flags are out of scope of this sketch.

Needs work:

• Function names
• API for umask
• Documentation
• Tests

[milseman]

I want to make sure we can make (even if slow) progress on this. @rauhul, have you thought any more about this?

[rauhul]

@milseman I think we need a good story about fcntl flags and if want to move the non-permissions bits of FilePermissions out into a different type. In the sketch as is, these are called FileModeOther as a placeholder. I think two other open areas are TimeSpecification and function naming.

[milseman]

Regarding Timespec, it might be interesting to see how we fit in with the outcome of https://forums.swift.org/t/se-0329-clock-instant-date-and-duration/53309

[rauhul]

@glessard would you be interested in reviving this PR?