Add support for TypeRep and Typeable

[jespercockx]

To properly implement some library functionality such as the fromException method of the Exception type class, we need a dynamic cast function from the Typeable module. Hence it would be good to have support for this in the agda2hs Prelude. However, an issue is that the implementation of the Typeable class uses the TypeRep type internally, which is a GADT. Hence actually implementing this properly is blocked on #242 (ping @zeroeightysix).

Possibly related: #207

[jespercockx]

I just realized that since TypeRep is an existing type, we do not need to actually generate any code for it, so supporting it might not require fixing #242 after all.