IEEE-754 recommended functions

As I was enjoying the entertaining read that is the IEEE-754 standard for floating point arithmetic, I noticed they recommend some functions (section 9.2), of which we currently lack:

• exp2m1: 2^x-1
• exp10m1: 10^x-1
• log2p1: log2(1+x) (might want to call this log21p, to match log1p?)
• log10p1: log10(1+x) (similarly, log101p?)
• rsqrt: 1/sqrt(x)
• compound: (1+x)^n (pow1p?)
• rootn: x^(1/n)
• various inverse trig-pi functions, e.g. atanpi: atan(x)/pi (somewhat confusingly, not all of these are mentioned in the table, although they are referred to in the text below).

There's also three different "pow" functions: pown for integer exponents, two for handling floating-point exponents pow and powr which seem to differ mostly in how signals are raised, so we could probably wait until #2976 is completed.