Categorize functions in std.math

Add a table to std.math so that the functions/constants are categorized similarly to std.algorithm.

I intend to ramp this up and perform similar modifications across the board assuming this gets positive feedback.

https://issues.dlang.org/show_bug.cgi?id=13555

Precedent: https://github.com/D-Programming-Language/phobos/commit/7fdfa02272a4bbfa65d17d7d85ba79480f2f9bd0

Results:

<img src=http://i.imgur.com/8s7RsMT.png/>