Clang JIT CPU Backend

Clang 16 now supports JIT. An interesting small project could be to create a `/cpu/self/clang-jit` backend that provides JITed tensor contraction kernels. If we see performance that is in the neighborhood of AVX or libXSMM, this could be a way to ship a faster CPU backend with fewer dependencies.

See Serac for reference:
https://github.com/LLNL/serac/blob/prototype/adjoints_with_internal_variables/tests/jit/basic_jit.cpp
https://github.com/LLNL/serac/blob/prototype/adjoints_with_internal_variables/include/JIT.hpp

(This repo comes from a member of Jamie's Smith team)