Implement exponentiation of BigDecimals by other BigDecimals

[JoonasC]

Is your feature request related to a problem? Please describe.
Exponentiation of BigDecimals by other BigDecimals is currently not supported, which forces a conversion to int value meaning precision loss.

Describe the solution you'd like
Implement exponentiation of BigDecimals by other BigDecimals.

[hakanai]

Having exp(x) would also make it possible to implement cosh(x), sinh(x), tanh(x) with not much extra effort.

The same is sort of true for sin(x), cos(x), tan(x) as well but you need complex number support for those. ;)