RedHat backend: Add {l,u}IdentifierToCeylon()
Add FullPackageName (RedHat’s ImportPath)

This is a bit messy, because for some reason the RedHat parser has a
separate token type for ’PIDENTIFIER’s, even though they’re lexed with
the same rule as ‘LIDENTIFIER’s and there’s no syntactical difference
between them.
Identifier backend tests: Make types more specific

Declare the types UIdentifier and LIdentifier instead of just
Identifier.
Add tests for FullPackageName backend conversion