Make WeekDay and Month enumerators public for time and chrono features

[Luni-4]

Without making Weekday and Month enumerators public for time and chrono features, it gets hard to retrieve week information when either a date or a timestamp is saved within a Postgres database.

This PR exports:

• Weekday and Month for time crate
• Weekday for chrono crate

Thanks in advance for your review!

[abonander]

SQLx only reexports the types that it actually needs to function. You should just add time or chrono to your dependencies and import from there.