format::Item documentation and ownership

[pitdicker]

From #852 (review):

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

This feedback seems useful to open an issue for #852 (comment):

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.

[pitdicker]

I have a commit that adds this as part of the experiments around a new formatting API.

[pitdicker]

Included in #1184.

[pitdicker]

Fixed in #1184.