Rename / alias __FlashStringHelper (e.g. to "FlashString").

We have a "friendly" alias for creating flash strings (the F() macro), but not for using them within a library. This is relevant for anyone who wants to create something that sub-classes Print (or Stream) and can print these flash-resident strings. For those people, it seems like it would be nice to have a nicer name than __FlashStringHelper (e.g. FlashString) and to document this somewhere.