Revamp `debug_printf` format string parsing

[LegNeato]

It appears our debug_printf format string parsing has some issues (see #51 (comment) and other comments).

I'd like to outsource most parsing to uucore's format string parser, but that is blocked on uutils/coreutils#6903 landing.

[LegNeato]

Think we need this too: uutils/coreutils#6946, as I want to wrap Spec to support vector stuff.