Wire in constraint tupleswip/wire-in-constraint-tuples

This wires in the definitions of the constraint tuple classes. The
key changes are in:

* `GHC.Builtin.Types`, where the `mk_ctuple` function is used to
  define constraint tuple type constructors, data constructors, and
  superclass selector functions, and
* `GHC.Builtin.Uniques`. In addition to wiring in the `Unique`s for
  constraint tuple type and data constructors, we now must wire in
  the superclass selector functions. Luckily, this proves to be not
  that challenging. See the newly added comments.

Historical note: constraint tuples used to be wired-in until about
five years ago, when commit 130e93aab220bdf14d08028771f83df210da340b
turned them into known-key names. This was done as part of a larger
refactor to reduce the number of special cases for constraint tuples,
but the commit message notes that the main reason that constraint
tuples were made known-key (as opposed to boxed/unboxed tuples, which
are wired in) is because it was awkward to wire in the superclass
selectors. This commit solves the problem of wiring in superclass
selectors.

Fixes #18635.

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Metric Decrease:
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Metric Decrease (test_env='x86_64-linux-deb9-unreg-hadrian'):
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Metric Decrease (test_env='x86_64-darwin'):
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