Extract non-portability constexpr math functions to new header

Extract non-portability constexpr math functions to new header

Summary:
[Folly] Extract non-portability `constexpr` math functions to new header.

This new header, `folly/ConstexprMath.h`, is specifically for compile-time-computable math functions. We start with `min`, `max`, `abs`, and `log2`.

Included substantive changes:
* Add new tests for `constexpr_strlen`, which remains in the portability header.
* Make `constexpr_min` and `constexpr_max` variadic.
* Make `constexpr_log2` tail-recursive, remove `const_log2` in `FixedString.h`, and move the related comment.

Reviewed By: Orvid

Differential Revision: D5739715

fbshipit-source-id: 29d3cc846ce98bb4bdddcc8b0fa80e4d32075fe0