Exponentiation syntax sugar

Summary:

Add a new binary operator for exponentiation. This involves two additions to the
language:

* Addition of new syntax for exponentiation (`expr ** expr` or `expr ^^ expr`)
* Addition of a new lang item for the corresponding operator trait `Pow`

[Rendered](https://github.com/SiegeLord/rfcs/blob/exponentiation/active/0000-exponentiation-sugar.md)
I do *not* think that the left-associative power operator is a good choice, but I do think that the choice of associativity needs a conscious decision. For the reference, the [PHP RFC](https://wiki.php.net/rfc/pow-operator#discussion) has a list of languages with the left-associative and right-associative power operator.