[#764] Switch to the new mutez subtraction operation

Description

Problem: the ithaca protocol introduces SUB_MUTEZ instruction and deprecates use of SUB instruction on type mutez. morley and lorentz should reflect this change.

Solution: implement SUB_MUTEZ instruction; stop SUB instruction from typechecking on mutez operands.

Related issue(s)

Resolves #764 (closed)

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