Contracts for Composable policy with arbitrary artifacts system

DO NOT USE IN PRODUCTION.
This implementation is in early development. It has not been reviewed or audited. It is not suitable to be used in production. Expect bugs!

The policy system needs to be backed with all kinds of setellite contracts to work properly. This package provides them.

• Predefined artifacts
  Provides a set of already writted basic artifacts to start with.
  They include some kinds of logical operators, mathematical operations, language constructions and so on.
  packages/contracts/contracts/pre-defined

• Policy handler entrypoint
  The main contract a third-party developer will interact with.
  packages/contracts/contracts/PolicyHandler.sol
  Contains 3 crusial methods:

1. set
   Creates the policy using its definition in final representation form (IR translator output, DSL compiler output).
2. evaluate
   Evaluates the policy agains provided variables. That is the main method to use when one wants to test if the operation (transaction, action, request, etc) complies with the policy created on step 1 (set). The variables must be provided in a specific format which can be fitted in with the help of variables package.
3. getVariablesList
   Used to obtain, enumerate or show to the client all the variables that must be filled for evaluate to be dispatched. Contains descriptors of the variables, their types and other metadata.

To use the policy system, one needs to deploy PolicyHandler.sol and all needed artifacts (ours or writted by third-party). After this, the following steps are considered standard:

1. Create policy
   Freshly deployed PolicyHandler do not contain any policy logic. Create the policy using DSL or other instrument that translates into FR, and then use the output to invoke set.
2. Prepare arguments
   If the policy contains any variables that need to be filled, use getVariablesList to get them and format their values in any convenient way (e.g. with the help of variables package).
3. Evaluate policy
   Use values obtained on step 2 to dispatch evaluate method. The policy may pass or reject - get the evaluation result as the method return value or via Evaluated(bool result, bytes32 rootNode) event.