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.

From my experience I've only seen it to be a problem if you are accessing data outside the range of the peek type.

eg:
```
void foo (real x)
{
  uint *p = *(cast(uint *)&x);
  if (p[2] == 0xDEAD_BEEF)  // Risk of being optimised as if (0)
  {
    /* ... */
  }
}
```

There should be a common union though (floatTraits?) that includes array accessors down to byte[T].