#124 Validate missed binding for participant instance variables

[Gary Brown]

When a performed choreography has a free variable, which is associated with multiple role types, where the role types are implemented by multiple participant instances, then validation is required to ensure that the variable is bound for all participant instances.

Current approach based on singleton participant type, where only one role associated with the participant type is required to be bound.

Two approaches are:
1) bind for all roles associated with the variable
2) bind for all roles associated with the participant type, or participant instances if defined

Can also support optional role type on binding, which could translate to any role required - although would still need the above validation for when role types are specified.