inverting exponents/roots in solving equations

[evykassirer]

right now if you get to x^2 = 4, it won't know to square root both sides.

operations that would be nice to invert on both sides:

• exponents to roots
• roots to exponents

these can be added to EquationOperations.isolateSymbolOnLeftSide (there's a TODO marking the place!)

in the future it'd also be great to do logs <=> powers -- though we don't support log functions currently (the only function node we support so far is abs), so that'd have to be done first but wouldn't be to big of a change I think