Shorten output for circular references

[overlookmotel]

Input:

const x = {};
x.y = {x};
module.exports = x;

Current output:

const y = {},
  x = {y};
y.x = x;
module.exports = x;

This output is longer than it needs to be. It's several bytes longer than the input.

[overlookmotel]

Same for deeper nesting e.g.:

const x = {};
x.y = {z: {x}};
module.exports = x;

[overlookmotel]

Possible implementation:

• Serialization functions return both a node object and a Set of records of circular dependencies.
• If the parent finds itself in that set of circular dependencies, it creates a deferred assignment x.y =.
• If not, it adds the set of circular dependencies to its own set of circular dependencies which it returns, to be handled by it's parent/ancestor.

This would also provide a solution to #75.

[overlookmotel]

This will be easier to solve once two-phase serialization is implemented (#426).