Bounded generics

[soutaro]

This PR is to implement bounded generics.

Now you can specify the upper bound of type parameters with < symbol.

class PrettyPrint[T < _Output]
  interface _Output
    def <<: (String) -> void
  end

  attr_reader output: T
end

type str_printer = PrettyPrint[String]   # OK
type int_printer = PrettyPrint[Integer]  # Type error

• The upper bound must be one of the class instance type, class singleton type, and interface type.
• Generic method can also have upper bound.

Todos

• Implement Writer
• Implement type name resolution in upper bounds
• Generic method type in Definition of generic classes
• Implement validation to detect circular dependency between type parameters
• Update docs