Fix CALL opcode in VM and compiler

[spencercorwin]

CALL opcode isn't implemented correctly in our Neo2 VM implementation. In the C# VM the alt stack is not copied over to the new execution context when the CALL instruction is executed. This leads to problems later on in the script execution when we try to call DUPFROMALTSTACK or any other alt stack operation where our compiler expected there to be values on the alt stack. Somehow we need to manage program scope differently or somehow bring the alt stack along with us to the new execution context.

See https://github.com/neo-project/neo-vm/pull/39.

Other updates related to this in #2293