app/lua53: catch up to lua 5.3.6

[nwf]

Diff reduction to 5.3.6, mostly guided by upstream.

Tested lightly using a subset of the expect framework; enough to verify that LFS continues to function, at least!

• [x] This PR is for the dev branch rather than for the release branch.
• [x] This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• [x] I have thoroughly tested my contribution.
• [x] The code changes are reflected in the documentation at docs/*.

[nwf]

Nobody here or #3414?

[nwf]

When I said "mostly guided by upstream", I meant that most of this was https://www.lua.org/work/diffs-lua-5.3.5-lua-5.3.6.html . The objbarriers are from that, FWIW.

[nwf]

Ah ha. After a day of debugging I have finally caught my mistake in this. Of course it was in the lundump.c bit where we've diverged from upstream. LoadFunction does not always (but does sometimes!) return its second argument, so it's important to re-assign, as in p[i] = LoadFunction(S, p[i], f->source);.

This now passes my expect harness's tests, modulo some local network shenanigans. But LFS got plenty of exercise throughout the run. :)

[nwf]

Will merge after #3479 lands.

[marcelstoer]

Will merge after https://github.com/nodemcu/nodemcu-firmware/pull/3479 lands.

Oh, that was a year ago 😄 Will merge now.