Flip signedness preference. Probably doesn't matter.
Check repr attribute consistency at check time, not translation.

Note that raising an error during trans doesn't stop the compile or cause
rustc to exit with a failure status, currently, so this is of more than
cosmetic importance.
Also require falsevariants to not ask for a discriminant
Improve assorted comments
fixup test to match changed error from check thing
Add yet another repr tag
Fix lint change to deal with Option<~T> etc