Refactor EntityToIdentifier stuff into ObjectToIdentifier

[stloyd]

Remove hardcoded dependency on Doctrine ORM, and allow instead to register transformer for every persistence type (ORM, ODM, PHPCR).

This is alternative approach to the #1244. Fixes #1199.

[stloyd]

Just additional note: if you remove (read: don't use) ORM in your project, the default usage of the entity_to_identifier form types will still work, as I'm creating an alias to the ODM version if ORM is not found (same goes with PHPCR, but this is last, so it's aliased only in case there is no ORM/ODM declared).

[arnolanglade]

yep, it better than mine! 👍

[stloyd]

@Sylius Guys, please share your opinions about this...

[jjanvier]

👍 easier to understand that @Arn0d's PR

[pjedrzejewski]

Thanks Joseph!