fix(doc): uv.spawn returns type

feat: add binding for semaphore to test synchronization

fix/thread: fix luv_getmtname trying to index a non-present metatable.

Merge pull request #767 from squeek502/libuv-1.51.0

Bump Libuv to 1.51.0

rework luv work vm storage

moves work vm storage into a garbage collected userdata.
this allows for each lua state to manage its own storage.

this removes the need for a global array of work vms.

Merge pull request #750 from zhaozg/master

bump LuaJIT to v2.1.1736781742

Merge pull request #696 from squeek502/scandir-gc

Fix garbage collection of scandir reqs

bump lua-compat-5.3 to 0.13

Annotate .gitmodules with branch (#693)

Bump version to 1.47.0 in CMakeLists.txt

docs: make uv.kill's signum optional