Add outcome expressions represented as terms

[HeinrichApfelmus]

This pull request adds a module DeltaQ.Term that implements a type O for outcome expressions represented as terms.

The term representation is accessible via a type Term. In contrast, the type O is abstract, but satisfies the properties required by the Outcome type class via normalization.

TODO

• Unit tests, refactor the generators to work in both cases
• Substitution via Monad (Term v)