This gives a more precise type signature to `magicDict` as proposed in #16646.
In addition, this replaces the constant-folding rule for `magicDict` in
`GHC.Core.Opt.ConstantFold` with a special case in the desugarer in
`GHC.HsToCore.Expr.dsHsWrapped`. I have also renamed `magicDict` to `withDict`
in light of the discussion in
https://mail.haskell.org/pipermail/ghc-devs/2021-April/019833.html.
All of this has the following benefits:

* `withDict` is now more type safe than before. Moreover, if a user applies
  `withDict` at an incorrect type, the special-casing in `dsHsWrapped` will
  now throw an error message indicating what the user did incorrectly.
* `withDict` can now work with classes that have multiple type arguments, such
  as `Typeable @k a`. This means that `Data.Typeable.Internal.withTypeable` can
  now be implemented in terms of `withDict`.
* Since the special-casing for `withDict` no longer needs to match on the
  structure of the expression passed as an argument to `withDict`, 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 `withDict` in
action, both in terms of `base` functions defined in terms of `withDict`
as well as in terms of functions from the `reflection` and `singletons`
libraries. The `T16646Fail` test case demonstrates the error message that GHC
throws when `withDict` is applied incorrectly.

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