Reword impl of partial_cmp for Ident, LocIdent

[giorgiga]

This rewords the implementation of partial_cmp for Ident and LocalIdent in a way that pleases clippy.

Note that the implementation was correct and that this does not change its behavior.

Of course another way to get rid of the lint error would be #[allow(clippy::incorrect_partial_ord_impl_on_ord_type)], but I think (more as a matter of principle than for any practical reason in this specific case) following clippy's advice and have partial_cmp call cmp is preferable to repeating the implementation, and... I also thing the annotation would be ugly 😃.

Anyway, in case they help clarify, here's clippy complains about Ident:

error: incorrect implementation of `partial_cmp` on an `Ord` type
  --> core/src/identifier.rs:60:1
   |
60 | /  impl PartialOrd for Ident {
61 | |      fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
   | | _______________________________________________________________________-
62 | ||         self.label().partial_cmp(other.label())
63 | ||     }
   | ||_____- help: change this to: `{ Some(self.cmp(other)) }`
64 | |  }
   | |__^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incorrect_partial_ord_impl_on_ord_type

and LocIdent:

error: incorrect implementation of `partial_cmp` on an `Ord` type
   --> core/src/identifier.rs:152:1
    |
152 | /  impl PartialOrd for LocIdent {
153 | |      fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
    | | _______________________________________________________________________-
154 | ||         self.label().partial_cmp(other.label())
155 | ||     }
    | ||_____- help: change this to: `{ Some(self.cmp(other)) }`
156 | |  }
    | |__^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incorrect_partial_ord_impl_on_ord_type

[yannham]

Thanks! Is this a new clippy warning? We have clippy enabled on the CI, so I wonder why we didn't catch that earlier.