Add exponentiation as a new operation

[jessealama]

Exponentiation occurs in some financial contexts, such as calculating a mortgage's monthly payment with a certain interest rate, principal, and term. There are undoubtedly other contexts in which exponentiation is a natural operation to use. Moreover, it is conceivable that the exponent of an exponentiation operation is not necessarily an integer, so it's not enough to say that exponentiation can be emulated simply by doing a few repeated multiplications.

[jessealama]

The new method should take one or two arguments.

• Single-argument variant: use Euler's constant e as a base; argument is the exponent.
• Two-argument variant: first argument is the base, the second argument is the exponent.