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	Compile instance declarations better
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This commit makes the type checker build better dictionaries
from instance declarations.  "Better" in the sense that if a
binding in the instance declaration mentions one of the other
methods of the same class, we use that method directly rather
than going via a (recursive) use of the dictionary.

This makes it easier to unravel recursive knots, and that
makes more inlining happen.  There's a long comment in
TcInstDcls.tcMethods

Makes the Monad instance for ST much more inlinable, and that
makes spectral/fibheaps go faster, among other things.