[#7] Rename `PairStruct` to `UnpairStruct` and use different `PairStruct` for PAPAIR

Description

Problem: PAPAIR and UNPAIR use the same structure (called PairStruct), but current structure is correct only for UNPAIR, because UNPAIR can produce multiple items on stack and hence can have multiple variable annotations, while PAPAIR can't.

So VarAnns inside PairStruct inside PAPAIR are always ignored.

Solution: Rename current PairStruct to UnpairStruct and use different PairStruct which exactly captures set of all possible PAPAIRs.

Related issue(s)

Resolves #7 (closed)

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