Don't include `{Event,HsUnix}Config.h` in source distribution

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Changed export list to create better haddock. Added eqT to exports.
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
Addition of `Nat1` and instances for it.
Apply Gabor Lehel's suggestions.