Add SelfReference + subtypes

SelfReference of This|Super|Outer|Package
Add backend support for SelfReference

With workaround for ceylon/ceylon-compiler#1658.

The compile functions don’t work yet because the lexer yields
LIDENTIFIER tokens (instead of THIS, SUPER etc. tokens).
Add backend tests for SelfReference

The compile test is ignored because the compile functions don’t work
yet.