[TM-87] Instructions should not get flattened

Description

Problem: a contract serialisation depends on its precise code. When we flatten instructions we lose information about an original version of the code.

Solution: remove flattening step and corresponding datatypes for AST representation. Where untyped Instr and Op were used, ExpandedOp is used now.

Related issue(s)

https://issues.serokell.io/issue/TM-87

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