Include slice information in connections.

 - All connection types (intermediate and reduced) now include
   information about the slices they are connected to.

TODO: Modify decoder computations to account for this new information.
flake8
Remove unnecessary function.
Remove decoder caching.

This feature is now provided by Nengo upstream.
Modify decoders to deal with slicing.

 - New decoder utils functions.
 - Reworked decoder compression to deal directly with keyspaces.