Add Identifier

As with FullPackageName, annotation identifiers have an annoying extra
token type (AIDENTIFIER). Why‽
Add tests for Annotation backend conversion + helper
Add Annotations (RedHat’s AnnotationList)

I’m starting to get very annoyed at all these magical phantom token
types. PIDENTIFIER, AIDENTIFIER, and now ASTRING_LITERAL and
AVERBATIM_STRING.

Annotations.children contains a workaround for a backend erasure bug
that looks like ceylon/ceylon-compiler#1728.
Add tests for Annotations backend conversion