Ledger types handlers

[kozross]

Closes #23 . This provides a structure to be expanded upon in later PRs, showing the three kinds of read-only accessors we provide for those types not designed to be constructed:

• An 'unwrapper', for single-field, single-arm products (or newtypes)
• A 'field accessor', for multi-field, labelled, single-arm products (or records)
• A 'destructor' for multi-arm sums (designed to take a Boehm-Berrarducci-style destructor function, as well as the data to be destructed)

Follow-up tickets will be made to expand this.