core: Add unwrap()/unwrap_err() methods to Result

core: Add unwrap()/unwrap_err() methods to Result

These implementations must live in libstd right now because the fmt module has
not been migrated yet. This will occur in a later PR.

Just to be clear, there are new extension traits, but they are not necessary
once the std::fmt module has migrated to libcore, which is a planned migration
in the future.