Directed coercions

This patch introduces a slimmer version of coercions, directed coercions,
which store fewer types within them. This more compact representation
considerably speeds up programs which involve many type family
reductions, as the coercion size no longer grows quadratically in
the number of reduction steps.

-------------------------
Metric Decrease:
    LargeRecord
    T12227
    T12545
    T13386
    T3064
    T5030
    T8095
    T9872a
    T9872b
    T9872b_defer
    T9872c
    T9872d
Metric Increase:
    CoOpt_Singletons
    T18223
    T9872a
    T9872b
    T9872c
    T9872d
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