Add String.replace

[ndmitchell]

It would be useful to have a replace function, with the semantics replace "ab" "foo" "testabab" == "testfoofoo". Such a function exists in text and extra, both named replace. Writing such a function on top of foundation is possible (just use list, extra.replace, back again) - but not very performant. For extra performance, you may wish to optimise the common case when the from string is a single ASCII char.