Make use of OCaml 4.14 features

[sim642]

In #1448 we bumped our OCaml lower bound from 4.10 to 4.14. This gives access to new language and standard library features that we could exploit. Below is a list of some, but more could be found by going to OCaml changelogs and @since annotations in Stdlib.

4.11

4.12

• Either module — could at least revert b4c0057, not sure if we want to use it for Printable.Either, etc.
• Hashtbl.rebuild — we've thought about using this to fix some potential GobView unmarshaling issues, but this needs to be investigated more (Fix load_run pruning entire solution in Gobview #651 (comment)).

4.13

• String.starts_with/ends_with — Use String.starts_with and ends_with from Stdlib #1523.

4.14

• 40 Seq functions — Switch from BatEnum to Seq #1437.
• tail-recursion-modulo-cons — we have some of our own list functions in GobList and possibly elsewhere.