More `codec` edge cases

https://github.com/paritytech/parity-scale-codec/issues/507 covers one but there are two more:

• Two codec attributes can be applied to the same variant with no warning,
• meanwhile a variant can have no codec attribute and there is no warning.

For example:

enum Foo {
    #[codec(index = 0)]
    #[codec(index = 1)]
    A,
    B,
}

This kind of bug can happen due to a copy/paste mistake or a mistake when resolving a merge conflict (as it did to me). When there are many attributes this kind of mistake can be hard to spot. In my case it resulted in the decoded value being different from the original value before encoding, which made for a fun debugging session.

Start by reviewing the linked issue 507 and reproducing the enum example with duplicate and missing codec attributes. The work is done when both invalid cases produce warnings and the existing codec behavior remains covered by appropriate tests.