(To do) Builtins: do something coherent for numeric helper functions

[lth]

spec/library/Math.html: there are numerous useful helper functions on numbers that are used throughout the spec (sign, copysign, isPositive, ...) that should get uniform treatment. They could be handled here, with the math functions, or they could be handled in the chapter on numbers. If so, functions that are currently global like isIntegral could well be treated at the same location.

I think it would be interesting to consider making some of these methods public, if they're good for us they're likely to be good for somebody else too.