Implement ES6/ES7 exponentiation operator

[classilla]

Most of this work had already been done in Fx42. We just need

https://bugzilla.mozilla.org/show_bug.cgi?id=1243858
https://bugzilla.mozilla.org/show_bug.cgi?id=1291212

This is already getting use in the field.

[classilla]

changeset:   347001:80683e82570c
user:        Andre Bargull <andre.bargull@gmail.com>
date:        Thu Oct 06 08:12:55 2016 -0700
summary:     Bug 1243858 - Disallow unary expression on left-hand side of expone
ntiation operator. r=arai

[classilla]

There are enough source changes between 45 and these sets that I'll just do them manually instead of importing them automatically.