Implement virtuality and constructor restrictions

[LPTK]

• allow fun and val members to be declared virtual; a signature without an implementation is considered virtual automatically
• check that overridden funs and vals are virtual
• forbid uses of this in constructors, including a class' initialization statements in the body and val RHSs
• make an exception to the above for each final (ie non-virtual) member which either is a val or is a lazy val or a fun that doesn't itself transitively use this

[NeilKleistGao]

I think all tasks are finished.