Fix bin/sort for constants

[pocke]

This PR partially fixes issue #918.

Previously bin/sort raises an error on a constant declaration in a class decl.

For example:

# test.rbs
class C
  def foo: () -> void
  C: String
  B: String
  A: String
end

$ bin/sort test.rbs
bin/sort:71:in `sort!': comparison of RBS::AST::Members::MethodDefinition with RBS::AST::Declarations::Constant failed (ArgumentError)
	from bin/sort:71:in `block (2 levels) in <main>'
	from bin/sort:68:in `each'
	from bin/sort:68:in `block in <main>'
	from bin/sort:62:in `each'
	from bin/sort:62:in `<main>'
Opening test.rbs...

This change fixes this problem. The test.rbs will be sorted as the following.

# test.rbs
class C
  A: String
  B: String
  C: String
  def foo: () -> void
end