syntax: Use UFCS in the expansion of `#[deriving(PartialOrd)]`
Update Partial/Total Eq/Ord terminology
auto merge of #18755 : japaric/rust/ord, r=alexcrichton

Closes #18738
cc #15689

r? @alexcrichton 
cc @cmr
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
Update tests accordingly