Add fixity for (==) and (/=)

[Dannyu NDos]

Given that they are instantized for `Bool`s, they are associative, so it
seems approvable to give them a fixity.

(Sidenote: For the monoid over (==) I suggested on last May, I stated that
it determines if there is an even number of `True`s, but that's wrong. It
determines if there is an odd number of `False`s.)
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