Method overrides can narrow a type from nilable to not-nilable

[Chuck]

...but they should not be able to. This must be an error:

	def equals(other as Object) as bool is override
# must be:
	def equals(other as Object?) as bool is override

[Chuck]

The test case for methods should be something other than .equals as that is about to become a cue. Here's a proper test case:

class A

	def foo(s as String?)
		pass


class B
	inherits A
	
	def foo(s as String) is override  # .error. cannot narrow
		pass


class P

	def main is shared
		pass

[Chuck]

Fixed in changeset:1832