Optimize clMul in Binary Common

[dhsorens]

Context

CompPoly/Fields/Binary/Common.lean contains this TODO above clMul:

optimize clMul, potentially using Karatsuba decomposition

clMul is currently a straightforward fold over 256 bits.

Proposed work

• Replace the current implementation with a faster approach (Karatsuba or another documented strategy).
• Keep the public API unchanged.

Acceptance criteria

• clMul/clSq behavior is unchanged on representative test cases.
• proofs and/or tests to show correctness