GeezerVison

I've taken the liberty of adding some assert_* functions just to be always sure that we don't mess up the indices. It's more for the future than for now since the assertions are simple. But I also felt that it'd be nicer to have a function call rather than lots of assert() one after the other.

I also removed some inline qualifiers as they are likely no more inlined (since we are calling an inline function with more than 1 or 2 lines more than once in the same function, I think it's better to let the compiler decide on the inlining itself).

• Issue: Refactor PyUnicodeError internal C helpers #127787