Fix mpfi pow function to correctly handle negative base with integer power.
Fixed error handling to not return numeric_limits<>::quiet_NaN if there is no NaN supported.
Fixed default pow implementation to handle integer args up to numeric_limits<uintmax_t>::max().
Fixed error handling in default pow implementation to return NaN for too large exponents (can't tell if they're integers or even or odd) and negative bases.
Fixed array subscript bug in cpp_dec_float.
Greatly increased pow testing.
Fixes #8809.