Identify and lower library functions

• Add LibraryCall node for representing opaque library calls.
  • Integrate into collections analysis and GCM to properly track collection flow.
• Add rewrite-math-expressions pass to rewrite einsum reductions into LibraryCall nodes.
• Lower LibraryCall to functions in hercules_rt in RT backend.
• Add wrapper for cublasSgemm.
• Rewrite juno_matmul test on the GPU.