Added `flexp2', `fllog2', `fllogb'; refactored and documented flonum testing

Note: With this refactoring, `math/utils' no longer depends on `rackunit'.

* (flexp2 x) computes (flexpt 2.0 x) but in about 1/3 the time for integer
  `x' using a lookup table. Written for exact argument reduction in `fllog2'
  after discovering that (flexpt 2.0 x) was the main performance bottleneck.

* (fllog2 x) computes (/ (fllog x) (fllog 2.0)) with near perfect accuracy.
  Invented an algorithm to compute it with at least 8 extra bits before
  final rounding; quite pleased with the result. Needed `fllog2' to ensure
  (fllogb 2.0 x) would be exact when `x' is a power of two.

* (fllogb b x) computes (/ (fllog x) (fllog b)) with better accuracy, and
  also handles limit values in a way that's consistent with the mathematical
  limits. When those are ambiguous, it's consistent with `flexpt', which
  follows IEEE 754 and C99. Otherwise returns +nan.0. See docs for details.

* `bflogb' is currently just for testing `fllogb'.

* Refactored FPU testing and documented it. So far, the only documented way
  to do it is by calling `test-floating-point', which runs a comprehensive
  deterministic+randomized suite of tests and returns a list representing
  failed tests. I'll document individual tests after I document flonum
  expansions and result/error functions like `fl+/error'.

* Added `fllog2' and `fllogb' to the flonum tests.