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 just checked, and in every language I tried this operator is right-associative. Is it a big deal to just make that be the rule? This wouldn't be the first binary operator in Rust that is right-associative (`=` is, although it is not overloadable). The `IndexSet` operator from the Indexing RFC would also probably to be right associative.