Additional String Processing APIs

[barnard-b (Benjamin Barnard)]

@Karl

In-place substitution only exists on NSMutableString

Hmm, it seems that most substitution (including StringProtocol.replacingOccurrences(of:with:options:range:)) is part of Foundation and not the standard library (Probably due to String.CompareOptions being a type alias of NSString.CompareOptions).

@davedelong

Sometimes I'll need to capitalize the first letter of a string, but not capitalize every word (like capitalized(with:) does):

capitalized(with:) is also in Foundation (because of Locale).

Since evolution does not cover Foundation, changes like this would be separate from a normal pitch.