Squashed 'vendor/compat-5.2/' changes from 898ede6..a1936f0

a1936f0 avoid identifier luai_Cast (also used by Lua 5.1 in some cases)

git-subtree-dir: vendor/compat-5.2
git-subtree-split: a1936f07f2093dea6243c169e4531970b75e38c6
Merge commit 'b5648ab1ac72360bebd37414e35e1afe74aa28c4'