Add DelimSpan to hold the 3 different spans of a delimiter

[dtolnay]

For getting the span of an opening punctuation or closing punctuation of a group, proc_macro only lets you use span_open() and span_close() if you still have the original Group that came in through the macro input. If you just have the group's Span, there is no possible way to split that into span_open and span_close after the fact (even though rustc is capable of doing this operation). You can't even fool it by doing let mut tmp = Group::new(Delimiter::Parenthesis, TokenStream::new()); tmp.set_span(span); tmp.span_close() -- this will just give you back span instead of the correct span_close.

Libraries sometimes want to keep track of delimiter spans separately from contents. An example of this is syn::MacroDelimiter -- https://docs.rs/syn/1/syn/enum.MacroDelimiter.html. Delimiters of a macro invocation cannot be Delimiter::None so representing them using the original Group would be too general. Today such libraries are forced to eagerly call span_open() and span_close() before dropping the original parsed Group, and then they need to store 3×proc_macro2::Span which is an excessive 36 bytes.

This PR adds a more compact 3×Span data structure which is only 12 bytes, the same size as a single proc_macro2::Span.