factor search tokenization out into a function

This ensures that the tokenization algorithm will remain in sync between
querying, indexing, and deindexing. The existing code had slightly
different behavior for querying, because it did not discard words with
>50 bytes. This was inconsequential, because >50 byte tokens are never
present in the index.

Signed-off-by: strawberry <strawberry@puppygock.gay>