Clean up `Coercible` plumbing
Currently, the way
Coercible is exported (with an data type in
GHC.Types and a built-in !TyCon exported in
GHC.Prim) works, but is confusing and should be cleaned up. Probably by exporting it directly from
Care needs to be taken that ghci and haddock display the data from modified built-in !TyCon; when I tried that a while ago it was displayed as a regular data type by one of them. If that is still the case and I didn't just do a mistake back then, this probably needs to be understood and fixed.