paths method performance

A lot of the `paths` methods are significantly slower then they could be.

I've found when invoked frequently (such as once per path in a large `glob` result) doing simple string manipulation is sometimes 10-100x faster (300+s to <10s in some instances). Primarily methods such as `paths.replace_extension` and `paths.split_extension`, and the underlying `paths.basename`. I think the primary reason being the creation of arrays/tuples only to extract a single string out of the array?

Start with the paths methods named in the issue, especially paths.replace_extension, paths.split_extension, and the underlying paths.basename, and reproduce their cost when called across a large glob result. Compare the current behavior with simple string manipulation while preserving the methods' existing results; done means the repeated operations show a substantial measured improvement.

Written by the indexing model from the issue text.