swift234

[SE-0091 Follow-up] Move global operators for non-generic concrete types into the type.

In various cases where we had global operators for non-generic
concrete types (such as String + String), move those operators into
the type. This should not affect the sources, but makes the exposition
of the library cleaner.

Plus, it's a good test for the compiler, which uncovered a few issues
where the compiler was coupled with the library.