Expose elementType of EnumSet and EnumMap

[Marcin Wiśnicki]

Third party serialization libraries and other tools need to know the
type of EnumSet elementType. Currently this field is package-private
and said libraries have to resort to non-portable reflection hacks.

Please add EnumSet#getElementType() and EnumMap#getKeyType() methods.

I've submitted this request to bugs.sun.com last month, where it
received Bug Id: 9006352. There was no further communication and that
bug is still not visible which doesn't surprise me as I've never had
any luck with bugs.sun.com even when submitting actual bug :(

PS. Please keep me CC'ed.