[#359] Turn `unpair` into standalone instruction

Description

NOTE: at the moment, this MR is based on top of !825 (merged)

• Added new UNPAIR instruction
• Renamed the UNPAIR macro to UNPAPAIR
• Updated macro expansion: expand UNPAPAIR macros to UNPAIR instructions.

Added optimizer rules:

• Optimize DUP; CAR; DIP { CDR } to UNPAIR
• Optimize DUP; CDR; DIP { CAR } to UNPAIR; SWAP

Also added a quality-of-life-improvement, to make it easier to write optimizer rules:

• The Seq constructor can now be used as a right-associative operator.

Related issue(s)

Resolves #359 (closed)

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