Finish out push, pop, shift, unshift implementation in PXStringValue

This is actually going to be deleted, but I wanted to have some record of this code in case it is needed later.
Move push, pop, shift, unshift, and reverse into PXArrayValue and out of PXExpressionArray

We want PXExpressionArray and PXExpressionObject to be minimal implementations of those types. Methods are already tied to specific types, so we only need these protocols to expose enough for what is needed by the byte code instructions.