Add id() method to algebra

[ilyavf]

var Car = DefineList({
   vin: "number",
   brand: "string"
});
Car.algebra = new set.Algebra( set.props.id("vin") );

var myCar = new Car({vin: 1, brand: "Nissan"});

// for an instance:
Car.algebra.id( myCar );                       // >>>  1

// for raw data:
Car.algebra.id( {vin: 2, brand: "Mazda"} );    // >>>  2

This can be helpful when working with both instances and raw data, e.g. in can-connection's map-deep-merge we need to know how to get id from data.

[justinbmeyer]

One consideration will be something like can-connect/can/map's id method. If it's a can.Map, it needs to read the properties differently.

We might need to also have an idProps ... so can-map can be read via .attr(). Of course, if can-operate land, algebra.id() should be able to use can-operate to read properties.

can-connect/can/map's id also needs to ignore observations if the id is being read during eventSetup or teardown via callCanReadingOnIdRead. This part might be avoided by using Observation.ignore.

[justinbmeyer]

One consideration is that CanJS should use valueOf to specify some unique representation.

Alternatively, this sort of id could be something symbols can provide.