Fixed outer loop counter, added 32 bit accumulation, new performance measurements

Fixed: end pointer for outer loop was calculated based on bytes, not samples. So loop had been run only half as often as expected. All performance measurements so far have been wrong.

Now the full 32 bit multiplications results are summed. Output is still Q15 format.
Activated "FAST" mode because now I think that this will work.

	modified:   firfilter/README
	modified:   firfilter/filtertest.c
	modified:   firfilter/firfilter.s