pared down mod2pi pull request

This is a counterpoint pull request, building on the excellent work of https://github.com/JuliaLang/julia/pull/4799. It's basically purely editorial – I just removed (and moved) things. For a first pass, adding three functions to Base seems like far too much. I decided to keep `mod2pi` of the three originally introduced functions since it's the most sensible one, modding down to a single turn of the unit circle. I'm really beginning to agree with the "pi is wrong" sentiment – if we used τ instead of π then `modtau` would be the unequivocal right choice.