derive Hash for most pub types that also derive PartialEq

[bruceg]

We needed to store Item alongside other data in a hash set, which requires all of the contained data be hashable. While I was adding the derive for that type, I looked for other pub types that could use it.

[esheppa]

Looks good. Nice to see the format::Item API being used.

@bruceg, please let us know if there are any documentation improvements needed in the format::Item and format::StrftimeItems API

[djc]

Probably want to backport this to 0.4.x, @bruceg do you want to submit that?

[bruceg]

A good example of how it can be used would be useful for documentation, ie feeding it in to format_with_items or what have you.

A bigger issue we had with this type was ownership. When parsing a string, we get items that reference the same lifetime as the string. When we later want to collect those items into a vec and store it for repeated use, we had to convert those into Item<'static> by boxing all the Literals and Spaces into OwnedLiteral and OwnedSpace. A better interface, that mimics other Rust types, might be to drop the owned variants from Item, but have a fn to_owned(&self) -> OwnedItem which has all boxed variants. I haven't looked at the underlying code to see if this makes sense for all your uses, though.

Funny you should mention 0.4.x. We use that for our releases, so we already did backport the change there. I will submit that after the weekend.