Make coroutine mappings weak (and document them).
Make coroutine mappings weak (and document them).

(x)pcall uses coroutines internally to allow yielding. The internal
mapping tables for those coroutines (and the initial coroutine that
started the pcall) should not keep the threads alive if the (x)pcall
yields and is never resumed again.

keplerproject/lua-compat-5.3,daurnimator/lua-spawn