Add DecQualifier (Spec’s MemberDecQualifier)

There’s no direct RedHat AST equivalent.
Dec: Add keyword attribute
TypeDec: Add qualifier and name fields

Renames GivenDec.typeParameter to name.
Add ClassDec (RedHat’s ClassLiteral)

Also adds RedHat conversion for abstract TypeDec (which I noticed was
missing), and some other minor changes.

(For the assertions in RedHatTransformer.transformClassDec(), see
ceylon/ceylon-spec#1076.)
Add tests for ClassDec, TypeDec
Add InterfaceDec (RedHat’s InterfaceLiteral)
Add tests for InterfaceDec backend conversion
Add AliasDec (RedHat’s AliasLiteral)
Add tests for AliasDec backend conversion
Add MemberDec, FunctionDec, ValueDec

RedHat’s MemberLiteral, FunctionLiteral, ValueLiteral.

Includes workaround for ceylon/ceylon-compiler#1784.
Add tests for MemberDec, FunctionDec, ValueDec