Span wrapper that provides equality and ordering based only on the lo/hi fields. Exists
because Span doesnβt impl PartialOrd/Ord due to the ctxt and parent fields being
unorderable. Useful when storing things in source code order, e.g. in a BTreeMap<OrdSpan, T>.
Specifies a field within one of the span formats. Provides const operations that (a) let a span
format be fully specified, and (b) check/read/write the field for/from/to a span.