[#663] Optimize PAIRN, DROPN and UNPAIRN

Description

Problem: Optimizer can't handle instructions working with variable number of arguments, and duplicating all rules for PAIR, DROP, UNPAIR for variable number of arguments is infeasible.

Solution: Expand PAIRN, DROPN and UNPAIRN into sequences of PAIR, DROP and UNPAIR respectively in the first optimization pass. Then merge what's left of those back.

Related issue(s)

Resolves #663 (closed) (unless I'm missing other instructions that could benefit from this treatment)

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