add comparison operators to must-use lint (under `fn_must_use` feature)

Although RFC 1940 is about annotating functions with `#[must_use]`, a
key part of the motivation was linting unused equality operators.

(See
https://github.com/rust-lang/rfcs/pull/1812#issuecomment-265695898—it
seems to have not been clear to discussants at the time that marking the
comparison methods as `must_use` would not give us the lints on
comparison operators, at least in (what the present author understood
as) the most straightforward implementation, as landed in #43728
(3645b062).)

To rectify the situation, we here lint unused comparison operators as
part of the unused-must-use lint (feature gated by the `fn_must_use`
feature flag, which now arguably becomes a slight (tolerable in the
opinion of the present author) misnomer).

This is in the matter of #43302.

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