Adding unit tests for fmath.PowerOfTwo(int64) bool

We use a simplified implementation of PowerOfTwo(...). We test the
following scenarios:

- Each known power of two 2^0 -> 2^63
- Each of the numbers 0 -> 10,000
- Each of the numbers 2^63 -> (2^63-10,000)
- Each of the numbers 0 -> -10,000
- Each of the numbers -2^63 -> (-2^63+10,000)
- 10,000 randomly generated positive integers
fmath + CombineInt32() HighInt32() LowInt32()

A trio of functions for combining 32 bit integers into 64 bit integers.
The methods take signed integers, but are safe for unsigned values with
appropriate type casting.