volk: cleaned up some sign compare warnings.
volk: r10 = sl --> removing duplicated register from NEON ASM proto-kernel.
volk: added a binary slicer that outputs int8_t data.

Only SSE2 simd version implemented.
blocks: adding VOLK support for some functions.

* multipy_const_vcc and _vff
* sub_ff
* float_to_complex
* binary_slicer

Also adds a min_XX block to complement max_XX.