Prioritise equalities when solving, incl deriveds
We already prioritise equalities when solving, but Trac #14723 showed that we were not doing so consistently enough, and as a result the type checker could go into a loop. Yikes. See Note [Prioritise equalities] in TcSMonad. Fixng this bug changed the solve order enough to demonstrate a problem with fundeps: Trac #14745.
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