[#123] Fix stack mutation in `MAP` primitive

Description

Problem: in our implementation `MAP` primitive discards all changes made
to stack elements deeper than the top one (which is set to current
iteration value). Incidentally this behavior was implemented with the
same error in Babylonnet. However in Carthagenet it was fixed and thus
our interpreter became non-conforming both to the spec and to the
reference implementation.

Solution: correctly keep track of stack changes in `MAP` implementation.

Related issue(s)

Resolves #123 (closed)

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