Improvements to std.math floor() / ceil()

Some changes are going on here:

1. Uses union instead to ensure that compiler doesn't emit any library calls here (seen a small speed up in benchmarking code).
2. Add float/double overloads, improvements might be negligible because isNaN and isInfinity checks perform (heavy?) conversions to long double.
3. Move common code in floor into floorImpl
4. Converted isNaN and isInfinity into generic functions.  This final change resulted in a substantial improvement of codegen.

We should really sit down and define aliasing semantics for D at some point. Otherwise, discussions like this are moot.