Use `0:` for named-fields debug of tuples and tuple variants

[ijackson]

Change the output when the user requests debug printing of a tuple struct in named fields syntax.

Previously we output _0: (for example), which isn't really right: Rust permits 0: , and the field is actually called 0.

This is a behavioural change, but I don't think Debug output is regarded as semver-relevant, so this shouldn't be a considered breaking change.