[stdlib] Refactor and optimize String.replace()

[martinvuyk]

Refactor and optimize String.replace()

CC: @JoeLoser another place where we need #3548

[martinvuyk]

Hi @ConnorGray this is why I wanted to have better iterator ergonomics in #3700:

s = String("123")
for c in s:
    ...

now gives the warning: warning: Use 'str.chars()' or 'str.char_slices()' instead.
I strongly dislike this. It should be an option to transform the _StringSliceIter.as_chars() or something along those lines (e.g. having CharIter(iter("123"))). Because this absolutely kills the pythonic default which is simple and makes it feel like writing Rust.