Remove ivory-generate.

It is not complete, it no longer works properly.

It should be revisited in the future (but probably
in a separate repository) with a better design
that would facilitate generation of more realistic
datasets (quality and scale wise). And should
also leverage what ivory does with it's testing
arbitraries which were almost 100% duplicated
within this code.
Merge pull request #231 from ambiata/topic/remove-generate

Remove ivory-generate.
Improve dictionary definition.

This change makes a number of improvements:
  - Definitions are no longer indexed in only one way, we
    have the core data and then a number of indexes for
    different situations.
  - Lots more tests over dictionary structure.
  - FeatureId becames a first class citizen in Definition
    this is importantant because a definition didn't make
    sense to pass around on its own before.
  - Sped up dictionary arbitrary (there was an exponential
    explosion in cases, so I have capped the number of base
    features).

Fixes #226.
Fixes #228.