Add meta generator

[dbuenzli]

Bring back the versioned meta generator tag (closes #324).

I made the test promotion as a separate commit. Btw. is there a way to promote things via a single diff. I had to run the tests, see one failing, promote, start over, etc. I'm sure I'm missing something.

Also I promoted a few things that seem unrelated to the change but is related to @trefis' spanning of sigs I think.

[dbuenzli]

The 4.02.3 failures are unrelated, apparently dune fails to compile.

[aantron]

Thanks.

IIRC, there is some command you could manually write to promote in one go, but there is no direct "support" for it at the moment.

[dbuenzli]

Thanks.

IIRC, there is some command you could manually write to promote in one go,

Eventually got around with:

for i in {1..XXX}; do cp `cat _build/default/test/html/_scratch/actual` `cat _build/default/test/html/_scratch/expected` && make test; done

But it's still very slow...