Implement underivable instances for Point

`Bind`, `Core`, and `R[1234]` can no longer be derived after the
addition of roles with GHC 7.8 (see [1], unfortunately spread across
three months of archives). Write these instances manually.

[1] http://www.haskell.org/pipermail/ghc-devs/2013-October/003003.html
Merge pull request #26 from bgamari/master

Implement underivable instances for Point