String methods for replacing a sub-string with another one in place

I've often needed to replace a sub-string within a String in place with a new sub-string. If the new sub-string is smaller than the sub-string its replacing, that can be done in place without any allocations.

I don't know if this is the right place to fill in a "feature request", but there is only one correct efficient way to implement such a method, and an RFC feels overkill for that.

The only thing I don't know is how to name it. We could name it String::replace_in_place and String::replacen_in_place but... if the new string is larger than the one it is replacing then an allocation will happen, so "in place" isn't 100% accurate, unless we consider "in place" to mean that it happens on this String, independently of whether it is reallocated or not.

Start with the Rust String API and the proposed replace and replacen behavior. Resolve the method naming and allocation semantics, including replacements larger than the matched substring, then document the agreed API and rationale through the RFC process.