Remove public enums

[0xTim]

Public enums are a nightmare with libraries in Swift, we should remove them

[dimitribouniol]

Seems like there are a few more left, like COSEAlgorithmIdentifier

[0xTim]

We might be able to get around this now with extensible enums - I think anything that's unlikely to be switched on by end users in most cases shouldn't be an enum, otherwise they can live as an extensible enum

[dimitribouniol]

Although I am very thankful we now have extensible enums, the other reason we don't want to be using them is so we don't choke decoding optional cases we didn't explicitly model out, which the spec makes copious use of 😅