syntax: Use UFCS in the expansion of `#[deriving(PartialOrd)]`
Update Partial/Total Eq/Ord terminology
Try to remove ty_nil, some kind of error in exhaustiveness checking
Complete the removal of ty_nil, ast::LitNil, ast::TyBot and ast::TyUniq

[breaking-change]

This will break any uses of macros that assumed () being a valid literal.
Update tests accordingly
Update the reference