[PATCH 0/3] Second step of 'show' rewrite

[Austin Clements]

This adds support for self-recursive message formatters, while
maintaining backwards compatibility with old style formatters.  After
this, each format can be converted to the new style individually and,
once they're all converted, a bunch of code can be deleted.

These three patches are independent and can be pushed in any order.  I
put them in a series because pushing them before any formatter
rewrites will simplify dependencies between the individual formatter
rewrites.