Syntax update

[soutaro]

Update RBS syntax based on the proposal.

• extension syntax is removed. RBS just ignores it. Use class or module syntax.
• RBS now allows writing multiple class and module declarations for one class/module.
• RBS now allows writing class/module/constant/interface/type alias declarations inside class/module declarations.
• Type name resolution became more compatible with Ruby's constant resolution. It resolves given relative type names based on class/module nesting structure. Note that the type name resolution does not refer inheritance and includes.
• overload def syntax allows to extend an existing method.
• super method type is now deprecated. Use overload def syntax.
• incompatible method attribute is deprecated and ignored.

Example

Before

module RBS
  def self.logger: () -> Logger
end

class RBS::TypeName      # Class decls under a namespace have to be out of module decls.
  ...
end

RBS::VERSION: String     # So is constant decls.

# Use `extension` construct to extend existing classes.
extension Integer (MyNumber)
  def to_my_number: () -> MyNumber  # Add new method

  def +: (MyNumber) -> MyNumber
       | super                                 # Use `super` to extend existing methods.
end

After

module RBS
  def self.logger: () -> Logger

  class TypeName      # Nested class declarations allowed.
    ...
  end

  VERSION: String     # So is constant decls.
end

# Re-open existing classes
class Integer
  def to_my_number: () -> MyNumber  # Add new method

  overload def +: (MyNumber) -> MyNumber  # Use `overload def` to extend existing methods.
end