Implement DCE

[krzema12]

Something is already inherited from JS backend, maybe it can be reused easily.
I also hope that DCE is a solution to MemoryError in MicroPython.

[krzema12]

It's not trivial - enabling it with the flag produces an empty file. It needs deeper investigation.

[SerVB]

I remember it worked at some point (when we had out-pir dir generated, at least). Probably a revision can be found.

Also, I remember Kotlin Team suggested that we don't reuse complex and smart KJS DCE that can still contain bugs but implement our own that just removes unused declarations but not code inside bodies.