Better docs
Drop handwritten Eq instance for Node
Document unimplemented functions
Allow other files to be used for benchmark data

We still use the system dictionary by default, but check the WORDS
environment variable first.

We also try to print a slightly more helpful error message if the
file can't be read.

--HG--
extra : amend_source : 7996f502998cda793b630898091e909d099a7414
Fix an awesome typo in foldrWithKeyWith
Generalize benchmarking across a whole map
Implement keys