Implement incremental plain text search

[SUPERCILEX]

Right now if you add characters to a plain text search, the entire database is queried again. It would be much more efficient to only start searching from the places where a substring was already found.

Implementation notes:

• To support backspacing efficiently, we should probably keep all the results from the first query and a stack of successive superstrings with indices pointing to the original results.
• Adding prefixes should be supportable by starting a search from offset - N where N is the number of prefix bytes.