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

Add access(2) constants to the constants table

fs_access is equivalent to access(2), but this function is useless
without the corresponsing access mode constants F_OK, R_OK, W_OK, and
X_OK. Let's add these constants to the table.

Yet another luajit.cmake unwind detection fix

Previously, CMake would only read up to the first NUL or control character (for me that meant it would only read "ELF" from tmpunwind.o).
This was making CMake think there was no `eh_frame`/`__unwind_info` string in the file even if there actually was.

Now, CMake skips binary data in tmpunwind.o and just reads all the ASCII strings from the file, which is fine for what we need to do with it.

This fixes:

    In file included from luv/deps/luajit/src/ljamalg.c:23:
    luv/deps/luajit/src/lj_err.c: In function ‘lj_err_unwind_dwarf’:
    luv/deps/luajit/src/lj_err.c:469:2: error: #error "Broken build system -- only use the provided Makefiles!"
      469 | #error "Broken build system -- only use the provided Makefiles!"
          |  ^~~~~

for me on Linux x86_64 when trying to build Luv

Bump luajit to latest HEAD of v2.1 branch