internal/ir+planner+compiler: add encoding for ir

This commit adds support for serializing/deserializing plans into/from
JSON. This allows us to compile policies out into JSON so that they
can be transpiled or interpreted in other environments.

To support these changes we have implemented custom marshaling on the
Block type and Operand (previously called LocalOrConst) type.

Signed-off-by: Torin Sandall <torinsandall@gmail.com>