std.math: add comptime_int handling for power-of-two functions

[Fri3dNstuff]

This PR implements handling for comptime_int inputs, and iN inputs, for the functions:

• std.math.floorPowerOfTwo
• std.math.ceilPowerOfTwo
• std.math.ceilPowerOfTwoPromote
• std.math.ceilPowerOfTwoAssert

Previously, all functions did not handle comptime_ints, and additionally the ceil- functions did not handle signed numbers, and asserted the input be positive - this PR changes the assertion to instead be a return of 1.

Generally, the current behaviour of the functions in the face of edge cases seems odd: why shouldn't the ceiling functions cope with non-positive inputs? And why does std.math.floorPowerOfTwo return 0 for a non-positive input: shouldn't that return an error? The existing behaviour of the floor function was not changed, due to it perhaps being dependent on, but it might be worth while rethinking.

[squeek502]

If you're going to change the implementation of ceilPowerOfTwo and friends, please provide benchmarks.

You can see how the current implementation was benchmarked here: #2426 (and some more context about 0 being disallowed as an input here: #2617)

(personally, I'd prefer this PR to do one thing at a time: make comptime_int work, and then make a separate PR for the behavior changes if those are wanted)