apitypes: depend on the standard library only

[charludo]

No description provided.

[sespiros]

Thanks. Note #2568 will need a rebase and separately I am thinking maybe it's actually better if we merged individual PRs in a stack instead of waiting for the whole stack to be approved because they might get stale.

One more thing to think about is that the encoding that produces transitionDigest is still an internal thing, so an internal refactor there would silently break deployed clients that build on apitypes. This is a general comment on the API design. So it's not only about the imports but also about the "contract" apitypes is supposed to capture.