Fixes some very misleading errors in the docs

• Use psacak instead of sais-drum to make the low memory suffix array construction less painful.
• Add some notes about 512 bit blocks to docs.

Lots of impactful and breaking changes:

• added second text with rank support implementation and made fmindex generic over new TextWithRankSupport trait.
• implemented low memory construction modes
• implemented optimized functions for searching multiple queries at the same time (per thread)
• other smaller features and optimizations

Initial release of this library.