test suite for mathematical functions

[StefanKarpinski]

This should be an implementation of much of Alan's knowledge — an executable encoding of how various math functions should behave, ensuring that the Julia implementation does the right thing and continues to do so. Should also help ensure portability, since the nasty corner cases are where different implementations tend to disagree.

[StefanKarpinski]

I'm reassigning this to v2.0 because I don't think it's crucial for a first release.

[ViralBShah]

This is a huge body of work, rather than just a test suite. Many of the questions are fundamental and often about making good default choices.

I would prefer to close this issue, and have specific issues for correctness and testing of various functions.

[ViralBShah]

Cc: @alanedelman