Updated FalseClass, NilClass, and TrueClass

[sampersand]

This primarily updates the unit tests for FalseClass, NilClass, and TrueClass to the newer system. It also:

• Removed public from the beginning of definitions (it's redundant)
• Removed def clone: (?freeze: true?) -> self, as that's defined on Kernel
• Collapsed redundant branches ((nil) -> false | (false) -> false is now just (nil | false) -> false. I decided not to do (false?) -> false, as the nil variant's important for the method definition, and doesn't just indicate an absence of a value)
• For NilClass, to_h now returns {}, not ::Hash[untyped, untyped]. It maintains parity with the other methods (eg def to_a: () -> [])

[soutaro]

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