[TM-377] Optimize packing of a constant

Description

Problem: there is one more optimization that our optimizer doesn't do: if a contract has PUSH instruction followed by PACK, it can replaced by a single PUSH that pushes the packed value.

Solution: add such a rule to the optimizer. Make it disabled by default because it seems to be potentially dangerous: instruction processing functions may treat constants somewhat specially and changing them to other constants may break such processing functions.

Related issue(s)

https://issues.serokell.io/issue/TM-377

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