Add C backend to Compyle

[aerorohit]

Add C backend to Compyle

Summary

• Adds a new c backend that transpiles annotated Python functions to C and compiles them via pybind11, enabling direct C
  execution without the Cython overhead.
• Introduces CBackend (in c_backend.py) — a CythonGenerator subclass that generates pybind11 binding code alongside the C
  source, and CCompile for single-function compilation.
• Extends Elementwise, Reduction, and Scan in parallel.py to support backend='c', generating templated C kernels with
  pybind11 bindings.

Unit tests

• tests/test_c_backend.py — new unit tests for CBackend/CCompile
• tests/test_cimport.py — new unit tests for Cmodule
• tests/test_parallel.py — extended parallel primitive tests covering the c backend