[#756] Add monadic `dfsInstr`

Description

In debugger we need an advanced dfs traversal of instructions. This MR changes our dfsInstr to support that.

The point is to make dfsInstr monadic. With such an interface, we get more flexibility in one place, e.g. our use case - enumerating instructions, is now easy to handle with State monad. But we slightly lose flexibility in other: CtorEffectsApp got less powerful. Anyway, we never needed it being that flexible.

Additionally I do some other minor additions that were found useful in the debugger project.

Related issue(s)

Resolves #756 (closed)

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