Redesign the type of magicDictwip/T16646-take-two

This gives a more precise type signature to `magicDict` as proposed in #16646.
In addition to making it more type safe:

* This allows the `magicDict` constant-folding rule to apply to classes with
  multiple type arguments, such as `Typeable @k a`. This means that
  `withTypeable` can now be implemented in terms of `magicDict`.
* Moreover, since the constant-folding rule no longer needs to case on the
  structure of the expression passed as an argument to `magicDict`, it no
  longer cares about the presence or absence of `Tick`s. In effect, this
  obsoletes the fix for #19667.

The new `T16646` test case demonstrates the new version of `magicDict` in
action, both in terms of `base` functions defined in terms of `magicDict`
as well as in terms of functions from the `reflection` and `singletons`
libraries.

This fixes #16646. By adding more tests for `magicDict`, this also
fixes #19673 as a side effect.

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