add a flag for generating variant traits for flags/enums

[jf2048]

GTK will probably not use this, but other libraries can enable it if they want

Fixes #1355

[jf2048]

cc @pbor

[pbor]

I have skimmed through the patch (I am not familiar enough with gir code to properly review)... can you show an example of how the syntax in Gir.toml would look like?

[jf2048]

It's just generate_variant_traits = "repr" or generate_variant_traits = "string" on the gir options (or on the type itself) and then you get traits for either serializing to int or string variant.