Stage improvements

This refactors the Stage struct to add a number of new features:

• stages now track their name (if set)
• stages now track their parent and root (image / stage)
• new Stages wrapper struct provides utility to get by name or index
• reduced Span debug output, now prints as (1, 23)
• additional testing

This is a mild breaking change for code that depends on the particular type signature of Dockerfile::iter_stages() or the Stage struct itself, as fields have been added.