derive: Avoid emitting PartialEq::ne for c-like enums

derive: Avoid emitting PartialEq::ne for c-like enums

`ne` is completely symmetrical with the method `eq`, and we can save
rust code size and compilation time here if we only emit one of them
when possible.

One case where it's easy to recognize is when it's a C-like enum. Most
other cases can not omit ne, because any value field may have a custom
PartialEq implementation.