Make Luv.TLS more usable with OCaml values

[aantron]

See https://aantron.github.io/luv/luv/Luv/TLS/index.html:

The interface is currently written in terms of nativeint, the type of pointer-sized raw values. You can store OCaml values in TLS by manipulating and registering them with the GC using Ctypes.Root. The conversions between unit ptr and nativeint are Ctypes.ptr_of_raw_address and Ctypes.raw_address_of_ptr.

A future version of this API is likely to handle GC roots internally, and be directly usable with OCaml values.