String search improvements

[dg-pb]

I have done some work on fastsearch.h module.

python/cpython#120025

PR was reviewed by few non-core-dev members and was subject to sufficient amount of iterations so that it is currently in stable state.

I have posted in coredev discourse, but without much luck.

So I thought maybe this is a better place to get some attention given that this is largely related to performance.

Apart from performance improvements, it does introduce consistency and simplifications that are helpful to move forward with further improvements (see String Search Overview, Coverage and API).

[markshannon]

It looks a worthwhile improvement, but I don't think anyone in our team is an expert enough in string searching algorithms to review it.

Trying pinging discourse again?

[iritkatriel]

CC @gsbrodal (an expert on string algorithms).