[#470] Added `PAIR n` instruction

Description

Problem: the edo protocol introduced many instructions for handling right-combed pairs, such as PAIR n, which takes n items off the stack and pushes a right-combed pair.

Solution: Add support for this instruction, including being able to typecheck it, interpret it, and parse it from the micheline/binary/michelson formats.

Related issue(s)

Resolves part of #470 (closed)

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