Issue Description:

Now that String's native storage is UTF-8, ending up with strings backed by UTF-16 storage can be more of a performance penalty (in particular, code that is optimizing for speed is now going to be optimized around a UTF-8 representation instead of a UTF-16 representation). As such, we should try to reduce the amount of UTF-16–backed strings we work with in general.

Unfortunately, a lot of core String functionality is provided by the Foundation overlay, and these methods all end up bridging to NSString in order to invoke the existing Foundation methods. Not only does this incur the bridging cost when going to Obj-C, but any strings created from these methods will be backed by NSString and thus will be UTF-16 if not ASCII.

I recognize that a number of these methods have complex behavior that would be a lot of effort to reproduce (in particular, anything that relies on the locale), but there's also basic methods that we could easily reimplement ourselves, such as components(separatedBy:).

As a trivial example, the following returns a UTF-16–backed string:

"héllo world".components(separatedBy: " ")[0]