use trait rather than fns

please note the snapshot-waiting unpleasantness. I'm
unable to use the traditional #[cfg(stage0)] mechanism
to swap the new style in for later compiler stages,
because macros invocations in method positions cause
the parser to choke before cfg can strip it out.

Parenthetical note: this problem wouldn't arise with
an interleaved parsing/expansion....
change to new trait style for method field refs

Per @pnkfelix 's suggestion, using a trait to make these
field accesses more readable (and vastly more similar
to the original code.