Add Posix.TTY

[athas]

It would be useful if Posix.TTY could be added to the Basis Library, as that would allow programs such as the following to work with MLKit: https://sigkill.dk/programs/noncanon.html

MLton has an implementation of this module, and since it is mostly just a wrapper on top of C code, I think it would be fairly easy to incorporate a solution based on the one in MLton.