Removed the false statement that all dictionaries are two-valued sequences.
Improved the error messages from generic interface contracts.

Previously, a failure during the first-order checks would print out the name of
the full contract for all the methods, while describing the value for just one
method.  This is both misleading, and incredibly verbose.  The new version
prints out just the relevant contract.
Avoid extraneous output from tests/generic/fallbacks.
Added a benchmark file for performance of gen:dict and gen:stream generics.