Support execution with transaction type-based dispatch

[jeromesimeon]

Currently, execution for clauses requires the user to specify the contract and clause. It would be useful to allow execution to be driven by the type of the incoming transaction (typically the clause request). The execution could match the incoming transaction type against the type signature of clauses.

This is also a useful item for integration with Cicero (#8), since Cicero already uses a form of dispatch.

[jeromesimeon]

There is some initial dispatching logic supported now (switch expression over CTO class names). This is used when generating the execution logic for integration with Cicero.