(1) enrich the trailer API (make trailer.h have simpler data structures
    and practical functions that clients can readily use), and
(2) make builtin/interpret-trailers.c, and other clients in the Git
    codebase use this new API.

I've done some more thinking/hacking and I'm realizing that changing the
data structures significantly as a first step will be difficult to get
right without being able to unit-test things as we go. As we don't have
unit tests (sorry, I keep forgetting...), I think changing the shape of
data structures right now is a showstopper.

Step (1) will have to be done without changing any of the existing data
structures.