Implement New object definition

[NeilKleistGao]

Related Issues:

• Iron out semantics of new object definition parameters and fields #166
• Implement virtuality and constructor restrictions #173

In this PR:

• Only generate fields for vals rather than lets
  • If a let binding is used in a method, we still generate a private field
• Only generate public getters for constructor parameters with vals
  • If it is in constructor(...) and has no val, the parameter is available only in the constructor
  • Add static function $unapply to non-plain classes(i.e., class ClassName(...))
  • Unfold let-bindings generated by UCS for $unapply method invocations
• Fix Code generation encountered an error: unresolved symbol if members only have type signatures
• Add virtual keyword for funs and vals.
  • If a member only has the type signature, it is virtual automatically
  • Require this when accessing the virtual members
  • Overriding non-virtual members is not allowed
• Add restrictions on constructors
  • Using this is forbidden in the constructor statements and RHS of vals, including directly and indirectly
  • Accessing non-virtual members via this is still allowed if the members do not refer to this

[LPTK]

I think a good test to add would be this:

abstract class Foo: Int -> Int {
  val x = f
  fun f = this(0)
}
//│ ╔══[ERROR] Cannot access `this` while initializing field x
//│ ║  l.6: 	  val x = f
//│ ║       	      ^^^^^
//│ ╟── The access to `this` is here
//│ ║  l.7: 	  fun f = this(0)
//│ ╙──     	          ^^^^
//│ abstract class Foo: Int -> Int {
//│   fun f: nothing
//│   let x: nothing
//│ }

Since it accesses this in an unusual way!

[LPTK]

Ok, I think I'm happy enough with the changes now!

[LPTK]

@NeilKleistGao please fix the conflicts and close the remaining conversations, then please merge (with a merge commit).