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

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.)