Mapping between STACK token and Symfony Token

[reenl]

In the current situation the stack token can only contain a single username. I would like to create some kind of mapper interface that converts the Stack token to a Symfony security token.

Use cases:

• Stack provides a (serialized) object that needs to be mapped to a Symfony token.
• Stack doesn't know the usernames, so it needs to map from any other kind of value.

Other use cases can be discussed within this issue.

[EDIT] The standard naming for this within Stack is TokenTranslator.