This speeds up self check by ~1.4%.  String equality is one of the top
five most common primitive function calls in self check.

We previously used a string comparison primitive that calculated the
relative order of two strings. Usually we only care about equality,
which we can do quicker since we can fast path using a length check,
for example.

I checked the CPython implementation of string equality in 3.9 (lowest
supported Python version) and 3.13, and both of them had a fast path
based on string object kind, and equality checks overall had the same
semantics.

Current CPython implementation:
https://github.com/python/cpython/blob/main/Objects/stringlib/eq.h

Tests for this were added in #19401.