If you know how to do this better please help.

The Day type needs to be serialized into a map, with stringified dates
as keys, and stringified items as values. Day is just a special case of
Vec<Day>, that is, a Vec of only one Day.

I cannot `impl Serializer for Vec<Day>` because these types are not
declared in the crate. So I implemented Days, that wraps a vec, but
users will have to use this instead and that's meh.

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>