Improve bench-memmem

Improve bench-memmem

Improve bench-memmem by replacing simple_memmem with a more efficient
implementation.  Add the Two-way implementation to enable direct comparison
with the optimized memmem.

	* benchtests/bench-memmem.c (simple_memmem): Remove function.
	(basic_memmem): Add function.
	(twoway_memmem): Add function.