New versions of IDNA UTS #46 have more flags

[SimonSapin]

Compare for example:

• https://www.unicode.org/reports/tr46/tr46-21.html#ToASCII
• https://www.unicode.org/reports/tr46/tr46-17.html#ToASCII

Currently we expose a public idna::uts46::Flags struct with only public fields, so adding another field is a breaking change. A new API design could make this more extensible, perhaps with a builder pattern.

Or on the contrary, should we make the idna::uts46 module private and only provide the configurationless IDNA algorithms defined in the URL Standard? (Oops, domain to ASCII has gained a beStrict boolean parameter. Though maybe this one should be exposed as a separate domain_to_ascii_strict function?)

[nox]

Flags is gone and we have Config now, where adding new flags isn't a breaking change anymore. Closing.