LuaJIT 3.0's new garbage collector design

Mentioned in the relevant github issue [1] that there's already a WIP implementation [2]:

"x64 only and not finished yet but it passes all the tests with assertions on and I'm seeing ~10% improvements on gc intensive benchmarks"

[1] https://github.com/LuaJIT/LuaJIT/issues/38

[2] https://github.com/achaulk/LuaJIT/tree/ngc