Opam installation updates

[jmid]

This fixes #345

When checking to see whether the ppx avoiding in #342 would fix the failing opam install from #345, I discovered that the #342's refactoring actually broke the 4.13 support just added in #340...

This PR thus

• writes out the Seq.equal definition contributed in OCaml 4.13 compatibility #340
• updates the opam install workflow to run on all changes

The install workflows are quite fast, so I favor protecting against such breakage again now that we actually support 4.13 and 4.14.

[jmid]

CI summary:

• The 32 bits test are failing as noted in 32-bit pre-setup fails #344
• A macOS 5.1 STM int64 ref test parallel asymmetric failed to trigger
• Cygwin tests are still ongoing (2 of them completed part 1 successfully at this point)

In the interest of restoring 4.13 support I'll therefore merge the PR.