Hash implementations

[ghost]

It'd be nice to offer Hash implementations for all of the date/time structs so that they can be used as keys in HashMaps.

[lifthrasiir]

I think NaiveDate can be reasonably used as a hash key. For other types, uh, I don't think they work. Can I ask you for the use case? (And sorry for delay!)

[ghost]

No problem! I didn't notice that NaiveDate worked; I was using Date<Local>. The main reason why it'd be nice to have is that I use Date<Local> and DateTime<Local> for all the times I parse from a user, and it seemed confusing that I need to call .naive_*() in order to use them as hash keys, because if I were to match the key, I'd have to re-cast them back into Locals, which might be ambiguous.

I think it'd be suitable if the Hash implementation just called .naive_date().hash() and ignored the time zone parts.

[pitdicker]

Fixed in #938.