Add String.replace_{first,last,all}

[dbuenzli]

This PR adds the functions string replacing functions discussed in #14137 it is built (two last commits) on top of the PR of search primitives #14381.

[nojb]

LGTM

A second core dev approval is needed before merging.

[Octachron]

The replace_all function is clearly useful and expected.

Overall, I agree with the labels for opposed arguments with a shared type.

I was a bit wary about the potential performance trap of replace_first or replace_last, but in fact the function are probably hard to use to emulate replace_all in an inefficient way (without resorting to testing physical equality). I am still not completely sure if those two functions would be really used a lot, but people would probably expect them to exist at least for symmetry's sake.

Globally, I agree that this PR should be part of the "String API improvement" bundle for 5.5 .

[dbuenzli]

I am still not completely sure if those two functions would be really used a lot, but people would probably expect them to exist at least for symmetry's sake.

I'm sure they will be, I can think of quite a few times where I had to replace a given suffix by something else (example here).

Thanks for the approval. Rebased.