Ad-hoc Concurrency Helpers

Regarding @murbard’s suggestions, I think implementing the MAKE_DFS instruction to mean labelling the emitted operation contextual (as discussed in Concurrency, BFS vs DFS (and a proposal)) instead of dfs clears up the worry these suggestions are addressing.

See my comment on the tzip discussion: https://gitlab.com/tzip/tzip/-/merge_requests/111#note_492008977