typeck::check::_match: Better error handling

Previously this was an Option::unwrap() which failed for me.
Unfortunately I've since inadvertently worked around the bug and have
been unable to reproduce it. With this patch hopefully the next person
to encounter this will be in a slightly better position to debug it.
Merge branch 'master' of https://github.com/mozilla/rust
Better handling on parameter lookup failure
middle::ty::str: Derive Show

This also adds Show instances for contained types.
middle::subst: Use custom Show for VecPerParamSpace
middle::kind: Use sty's Show instance instead of {:?}
middle::typeck::astconv: Use Show instead of {:?}
typeck::infer::region_inference: Use Show instead of {:?}
middle::typeck::astconv: Use Show instead of {:?}
syntax::ast: Derive lots of Show impls