Enum generated FromStr improvements

[mzeitlin11]

After using this, I realized a couple issues

• Writing from_str(s)? reminded me () as the error type for FromStr is terrible since it doesn't impl Error or Display.
• Enums which fallback to Unknown can have an infallible FromStr.
• Fallback behavior for from_str and deserialization did not match.

There's another question which doesn't need to be answered here around usage of Unknown as a fallback so that added enum variants are not a breaking change for parsing.

The openapi code currently uses the completely unscientific value of 12 variants for determining when Unknown should be inserted - there's probably an argument for using Unknown on smaller enums than that (or always, but that might start to hurt ergonomics for pretty stable enums).

[arlyon]

There's another question which doesn't need to be answered here around usage of Unknown as a fallback so that added enum variants are not a breaking change for parsing.

I have no strong opinions here. I am tempted to say that if you update your stripe API version you should make sure this library is updated to handle it but I don't think that adding variants is considered a breaking change... I think we could bring the threshhold down a little, perhaps down to 4 or 6, or maybe always insert Unknown for 'regular' enums (as opposed to tagged variants with associated data). I have a feeling most of the churn will come from 'this field has n string values' rather than fundamental differences in response type.

[arlyon]

I will open a ticket on the project and lets move on.