[TM-354] Optimizer depends on tree structure of its input

Description

Inside test/Test/Optimizer.hs there's

unit_UNPAIR_DROP :: Assertion
unit_UNPAIR_DROP = do
  optimize @Stack1Pair @Stack1 (DUP `Seq` (CAR `Seq` (DIP CDR `Seq` DROP))) @?= (DUP `Seq` CAR `Seq` DROP `Seq` CDR)

If we change the order of brackets to

  optimize @Stack1Pair @Stack1 (((DUP `Seq` CAR) `Seq` DIP CDR) `Seq` DROP) @?= (DUP `Seq` CAR `Seq` DROP `Seq` CDR)

then test consequently fails meaning that the sequence passed to optimizer depends on its tree structure which should not. There's already written flattenning function Michelson.Optimizer.flattenSeqLHS which consequently needs to be refactored. So this MR resolves the problem of optimizer's tree-dependence.

Related issue(s)

https://issues.serokell.io/issue/TM-354

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