Dataflow: add language-specific hook for breaking up big step relation

[owen-mc]

Python and Ruby have been (ab)using CastNode to do this. Go now has a use-case as well, so I have made a proper language-specific hook.

[aschackmull]

Looks reasonable to me. Only minor comment is that flowCheckNodeSpecific isn't a very good predicate name, I think. Perhaps neverSkip, neverSuppress, neverHide, or something like that would be better. The qldoc is also a little off since FlowCheckNode is a rather internal name and not really a good name to mean anything outside its small private context. Here's my alternative take on the qldoc:

Holds if `n` should never be skipped over in the `PathGraph` and in path explanations.

(And having just written that qldoc, I think I'm leaning towards neverSkip as the predicate name unless someone has a better suggestion.)

[smowton]

neverSkip is pretty vague if you don't know this has to do with path explanations. I don't have a good answer right now though.

[aschackmull]

neverSkip is pretty vague if you don't know this has to do with path explanations. I don't have a good answer right now though.

neverSkipInPath, neverSkipInPathGraph, includeInPath, includeInPathGraph?

[owen-mc]

I've gone for neverSkipInPathGraph.

[hvitved]

LGTM

[yoff]

Nice, thanks for doing this. Abused semantics is always a liability, so getting rid of that feels very good to me 👍