*safe* coerce, for regular and existential types

[Derek Elkins]

Throughout this message you imply, if not outright state, that Dynamics
requires unsafeCoerce/unsafePerformIO.  This is simply not the case. 
GHC implements Dynamics with unsafeCoerce, or did last time I checked,
but it can easily be implemented using only existentials. (I presume
that this decision was made either for efficiency, simplicity, and/or
simply that another (readily useable) technique was not known when the
library was made.)

Anyways, as I have often mentioned, "A Lightweight Implementation of
Generics and Dynamics" has an unsafePerformIO/Coerce free implementation
of Dynamics as well as Generics as the title suggests.