Add fast-path for memset, memcpy, and memmove

[daniel-grumberg]

The additional "calls" to __klee_handle_mem{set|cpy|move} are intercepted by the klee SpecialFunctionHandler and implement the intended functionality directly in concrete cases. The associated PR in klee is at https://github.com/klee/klee/pull/1341 which needs to go in before this gets merged.