[RFC] Implement "unsupported" tag entries

[cyphar]

This came up in #48 because implementing generic comparisons means we have to come to terms with the limitations of certain spec generators. In particular, because most tar archives don't have a . entry and they don't store directory sizes (see #77) there isn't a sane way of handling this implicitly -- we don't tag a manifest based on where it came from.

A solution that I think would work is if we always set the requested keywords on every object (which would ensure that you don't get Missing errors for keywords in #48). But if a keyword is not supported for that object we can set the value of the object to a special value like \x00 (which is not valid for any keyword).

This would be an extension of BSD's mtree(8), but I think it's a fairly safe one because all of the old code will continue to work (because BSD's mtree(8) doesn't support tar archives anyway).