Support caching the suffix array.

If the same old data is passed to bsdiff repeatedly, allow caller to
cache the suffix array, this is used by imgdiff in zip mode.

Bug: 34281147
Test: =time -v imgdiff -z Chrome-ORF74B.apk Chrome-ORF76B.apk Chrome.imgdiff
Change-Id: I0a69fc2d0b1a0d5791f78d0380df957981318daa