Correctly save relations

[nechaido]

At the moment the only way to check whether an entity has many-to-many relations is to check the indexes property.

It would be good to have a dedicated relations property.
Alternatively references property could be changed to include info about the reference.

Additional context
metarhia/metagram#2 (comment)

[tshemsedinov]

Example:

relations: Set(2) {
  { to: 'Company', type: 'many-to-one' },
  { to: 'Address', type: 'one-to-many' }
},