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)
In usual mathematical notation it's [right-associative][wiki]:

> Without parentheses to modify the order of calculation, by convention the order is top-down, not bottom-up:

> ![exp](http://upload.wikimedia.org/math/a/c/f/acfa11124b1bd069678048a3358eabba.png)

I think this is a compelling argument in favor of following the same convention.

[wiki]: http://en.wikipedia.org/wiki/Exponentiation#Identities_and_properties