Return a userdata wrapper around the uv_req_t to allow it to be garbage collected before the program exits. Previously, the returned userdata held a reference to itself in the Lua registry, meaning it would never be able to be garbage collected until the process ended.

This reverts commit 0e4a895, which attempted a workaround for the same underlying problem that introduced a use-after-free.

From the clang docs:

> By default, non-fatal checks are those enabled by UndefinedBehaviorSanitizer, except for -fsanitize=return and -fsanitize=unreachable.

This was doing pointer addition where i could be >= LUV_THREAD_MAXNUM_ARG and then subtracting afterwards. By doing the subtraction first, the number is properly bounded within 0..LUV_THREAD_MAXNUM_ARG

See #694 (comment)

Fix garbage collection of scandir reqs