Stdlib: Add `String.map` and `String.flatMap` functions

[alex-snezhko]

Mapping the characters of a string is sometimes useful to transform a string in a similar manner to other collections.

[spotandjake]

String.map was added in #1864

[spotandjake]

String.map is implemented as String.map(Char -> Char, String) -> String would the expected implementation of flatMap be String.flatMap(Char -> String, String) -> String?