Cythonize _linker.py

Ideally this should be P0 because numba-cuda uses Linker and this could be in a hot spot. However, by lowering to Cython it means cuda.core has to bump the lower bound for cuda.bindings from 12.6 to 12.8 (the first cuda.bindings release that has nvJitLink bindings included). We might want to wait for a few more months before pushing this?

Update Context.handle and Linker.handle to return 0 (not None) when closed.