Most of the work here is porting the LAPACK interface from Cython to plain C++. This is something I wanted to do anyway to make use of C++ templating facilities: the code is noticeably shorter in C++.
This change removes the only use of Cython in JAX. It also removes the need for a build-time dependency on Scipy, which we only needed for Cython cimport reasons.
When using C++, we most likely do not want to fetch LAPACK and BLAS kernels from Python. Therefore we add another option: we define the LAPACK functions we need using weak symbols where supported; the user can then simply link against LAPACK to provide the necessary symbols.
Added a jaxlib:cpu_kernels module to facilitate using the JAX CPU kernels from C++.
PiperOrigin-RevId: 394705605
