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 suggest just making it an error to have an expression where associativity makes the difference.

This simplifies the language, and it should be very rare anyway.