Pattern synonym that provides `Typeable` constraint

[Baldur Blöndal]

https://github.com/ghc/ghc/blob/4bb54a4522d44a81b2c47233f482
52bd73c38279/testsuite/tests/polykinds/T14270.hs
https://github.com/sweirich/dth/blob/2bb0c9aa9adb514b787ea56
295419fae55dc3734/examples/dynamic/DataDynamic.hs

This is a pattern synonym to construct but more importantly to deconstruct
an explicit `TypeRep a` into a `Typeable a` constraint

> asTypeable :: TypeRep a -> Dict (Typeable a)
> asTypeable rep =
>   withTypeable rep
>     Dict
>
> pattern Typeable :: () => Typeable a => TypeRep a
> pattern Typeable <- (asTypeable -> Dict)
>   where Typeable = typeRep

Similar discussion for SingI [1], same benefits:
+ (Nice) direct alternative to CPS `withTypeable`
+ Reusable in other pattern synonyms

[1] https://github.com/goldfirere/singletons/issues/273
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