Move allElements onto LibraryReader

[natebosch]

This was renamed in #228 to be more friendly as we expose it - but a top level method is still a little odd.

Maybe we should stick this on LibraryReader instead...

[natebosch]

Hmm, I wonder if Generator.generate should get a LibraryReader rather than a LibraryElement for the same reason we pass a ConstantReader rather than DartObject in generateForAnnotation...

[natebosch]

Making it take a LibraryReader would also make the Element -> LibraryElement change breaking and way more obvious, which was a concern in #223 (comment)

Any opinion @matanlurey ?