Implement CheckHyphens

[nox]

Fixes #483

This change is 

[nox]

I need to write a test but lunch time is approaching. :)

[nox]

I made idna::domain_to_ascii not check hyphens, and idna::uts46::to_ascii continue to check hyphens by default. Not sure that this is the right trade-off to make.

[SimonSapin]

UTS 46 does not specify defaults for its flags, but checking hyphens is what previous versions did:

• https://www.unicode.org/reports/tr46/tr46-19.html#Validity_Criteria
• https://www.unicode.org/reports/tr46/tr46-17.html#Validity_Criteria

The URL standard explicitly sets CheckHyphens to false. So yes, I think these are the right defaults.

I think I’d rather not add a new Config type in the pubic API as long as Flags also exist. What do you think of keeping it private for now? I’ve filed #486 about cleaning this up on the next breaking change batch, and referenced it from #463.

[nox]

I don't think making Config public is an issue, even when Flags goes away in a future version Config will still be here. I tried to make the change anyway, and I can't use pub(crate) because that stabilised in 1.18.0.