Subplanner support

[Arachnid]

This PR adds support for 'subplanners' - nested executions of weiroll that share their state arrays but have distinct command lists.

Example usage:

const planner = new Planner();

const subplanner = new Planner();
const sum = subplanner.add(Math.add(1, 2));

planner.addSubplan(SubcommandContract.execute(subplanner, subplanner.state));
planner.add(Events.logUint(sum));

const { commands, state} = planner.plan();

Return values from one subplan can be used in the parent scope, and even in other subplans. The planner constructs command lists for subplanners and inserts them into the state.

[github-actions]

size-limit report 📦

Path	Size
dist/weiroll.js.cjs.production.min.js	31.03 KB (-6.1% 🔽)
dist/weiroll.js.esm.js	29.17 KB (-6.4% 🔽)