[superseded] add math.copySign

[ringabout]

ref nim-lang/RFCs#92

C99 function
http://www.cplusplus.com/reference/cmath/copysign/

Python:
https://docs.python.org/3/library/math.html#math.copysign

Crystal:
https://github.com/crystal-lang/crystal/blob/5999ae29beacf4cfd54e232ca83c1a46b79f26a5/src/math/math.cr#L95

Dlang:
https://dlang.org/library/std/math/copysign.html

Rust:
https://doc.rust-lang.org/beta/std/primitive.f64.html#method.copysign

[ringabout]

sgn is ugly
https://stackoverflow.com/questions/1986152/why-doesnt-python-have-a-sign-function
And copysign is more useful
https://bugs.python.org/msg59154

[timotheecour]

superseded by #16406