Add rand function family

[myclevorname]

Currently, the only two places with rand implementations are my implementation in Haskell and a Python implementation in awesome-ti-docs. It would be helpful if the rand family were included in tifloats_lib_rs.

[rpitasky]

Fortunately, I already have a rust implementation (though it needs to be cleaned up).

I would also like to implement the CORDIC ops (exp, sin, cos, etc) here someday, but I don't know the specifics there (though TI has previously shared some papers to help with this... somewhere)