bslim_printer: Support printing StringRef values.

Printing `bslstl::StringRef` values was not previously supported
because the type does not contain a `print()` method and also does
not declare the `bslalg::HasStlIterators` type trait due to not
fulfilling the requirements.  Since `StringRef` values should print
enclosed in quotes similar to `bsl::string` values, support printing
them directly and defer to `StringRef`'s `operator<<` implementation.