STORE ops should not leave the value on the stack

[p7g]

Rather than having the majority of store operations require a following OP_POP to remove the value from the stack, the store instruction should not leave a value on the stack. If we need the value to be left there (i.e. assignment expression), it should be implemented as a duplication of the top of the stack and then storing one of those values.

This should have a slight performance improvement, and will reduce the amount of bytecode needed in most cases.