Add support for and test against Lua 5.5

[squeek502]

Intended to close #787

However, Lua 5.5.0 seems to somehow introduce regressions in our thread and work tests. Some of the thread tests hang, and the test threadpool test of test-work fails with

Uncaught Error: ./tests/test-work.lua:24: attempt to perform arithmetic on a nil value (local 'n')
stack traceback:
        ./tests/test-work.lua:24: in function <./tests/test-work.lua:23>
        [C]: in field 'run'
        ./lib/tap.lua:61: in function <./lib/tap.lua:48>
        [C]: in global 'xpcall'
        ./lib/tap.lua:48: in upvalue 'run'
        ./lib/tap.lua:146: in local 'tap'
        tests/run.lua:23: in main chunk
        [C]: in ?

Need to figure out what's going on. The documented incompatibilities don't seem relevant on first glance.

[truemedian]

The list of incompatibilities isn't very descriptive, but the clang-asan test fails that we're getting a null string from dumping the function and the rest of the tests are failing as if the thread didn't start with the function we expect.

So this seems to have been a change with respect to how lua_dump works.

The implementation of string.dump in Lua 5.5 has an additional branch that we likely need to reproduce: https://www.lua.org/source/5.5/lstrlib.c.html#writer

[squeek502]

Thanks, your comment got me pointed in the right direction. luv_thread_dumped should now be much more resilient against future changes to the details of lua_dump as well.