Deprecate Math.NumberTheory.Powers

[Bodigrim]

I've been long suspecting that {integer,natural}{,Word}Power functions are no better than (^) in contemporary GHC and base. Several months ago I have written benchmarks (branch wip-benchmarks). Results (https://github.com/Bodigrim/integer-logarithms/blob/wip-benchmarks/benchmark/results.csv) show that:

• Depending on bit pattern of an exponent, integerPower appears slower or faster than (^). Typically it is 10% faster for exponents 2^n, 10% slower for exponents 2^n-1 and 30%-60% slower for exponents 2^n+1.
• For Natural it appears that (^) is almost always faster (up to 60%) than naturalPower.

Taking into account such evidence, I find reasonable to deprecate {integer,natural}{,Word}Power and use (^) instead. I have already done it in arithmoi, without any negative consequences.