Name changes and addition of gcastWith and (==) to Data.Type.Equality.

For more info, see
http://ghc.haskell.org/trac/ghc/wiki/TypeLevelNamingIssues
Name change `CoercionT` to `CoercionType`, and addition of `repr`.

`repr` converts a `(:~:)` to a `Coercion`. Name changes are as discussed
on http://ghc.haskell.org/trac/ghc/wiki/TypeLevelNamingIssues
Apply Gabor Lehel's suggestions.