Merge pull request #1 from nomeata/master

Simpler type signatures
eta-expand the unwrap functions for foldl

Previously it relied on GHC to eta-expand the loop function,
but it did not always happen.

Joachim Breitner kindly pointed this out (thanks!)
http://www.haskell.org/pipermail/ghc-devs/2014-January/003940.html
readme.md: add a note about migration
test.hs: add serializeTree example
Make filter also a good consumer