structured compare

[graydon]

There is presently no support for comparing structured values (equality-wise,
order-wise or otherwise). Setting aside, if we may, the thorny question of how
we spell the deep vs. shallow comparison operators, it would be nice if we
could get some structured-comparison operator working.

[pcwalton]

Looks like we have the beginnings of this in commit 9f0a6c2.

[graydon]

Shifted to rustc. Issue has to be re-addressed there.

[graydon]

Implemented at least as far as rec/tup/tag in 7f2398e.