Export consts across crates

rustc still does not have the ability to read constants from external crates. Trying results in the following error: "upcall fail 'Assertion ccx.consts.contains_key(did._1) failed', /media/src/rust-tinderbox/srcdir-snap-stage3-x86_64-unknown-linux-gnu/src/comp/middle/trans.rs:5089"
