Add GHA workflow

[erikmd]

FYI @yurug @AltGr

[erikmd]

@yurug and @AltGr

I added a GitHub Actions workflow in learn-ocaml-corpus as requested at our last meeting.

Good news! → the learn-ocaml-corpus successfully builds from source with all versions of learn-ocaml from 0.12:
namely, the workflow tests {0.12,0.13.1,0.14.1,master}.

Downside: the 0.12 test is significantly slower (23' > 2'), and requires a workaround to overcome an old bug (ocaml-sf/learn-ocaml#305).

I don't know exactly why the 0.12 build is so slow.

Anyway, what solution do you vote for?

1. Keep this PR as is, testing 0.12 likewise for the master branch and each PR test
2. Keep this PR almost as is, but disable 0.12 testing on pull_request to earn time (while keeping the ability to trigger it manually)
3. Drop 0.12 testing in learn-ocaml-corpus, and drop 0.12 compatibility in learn-ocaml

AFAIAC, I'd be equally up for 1. or 2., considering that we can switch to solution 3. anytime…

WDYT?

[erikmd]

@yurug just voted for option 2. as well in our last telco.

So I'll refactor the PR accordingly.

[erikmd]

FYI @yurug, with @AltGr we decided to drop the integration test with versions < 0.15.0
because versions upto 0.14.1 did have the vuln. fixed by PR ocaml-sf/learn-ocaml#560,
hence this incentive to migrate if need be.
Of course, we'll try to keep backward-compat. for all versions ≥ 0.15.0.
I plan to fix this PR and merge it today.

[erikmd]

The CI just approved ✔️, so as promised, let's merge this one