Upgrade to lua 5.4

[hugopeixoto]

I've added some tests to the boostrap_* functions.

As mentioned here, I've had to remove setenv/getenv. I have also removed bootstrap_unpack, as it seems to have been removed from the stdlib.

I re-ported the other functions from 5.4 except inext, which I can't find. I had to change some things in there. I may need to add some tests there, not sure how it is supposed to work.

[whitequark]

Looks like CI fails.

[hugopeixoto]

This seems to be because Lua 5.4 is still in beta, and liblua5.4-dev is only available in experimental.

I could wait for 5.4 to be released, but making this work with 5.3 is simple. I'll create a PR for 5.3 and update this one if that gets merged to just contain the changes between 5.3 and 5.4.

[whitequark]

Sure.