librustc: Implement coercion for traits.
Add tests for trait object coercion.
std: remove some test warnings
std: result::collect to take an iterator, add option::collect
Renamed ClonableIterator to CloneableIterator
Rename pkgid variables
Rename PkgId to CrateId
Rename uses of PkgId to CrateId in librustpkg
Update Docs to use crateid