Allow creating multiple roles on the same line.

[damnMeddlingKid]

Currently if we try to make multiple assignments of roles single line we get an error.

action Init:
    accounts = [Account(name=customer), Account(name="Bob")]

> panic: Creating multiple roles in single step is not supported yet

The work around is to assign roles on multiple lines which is kind of awkward

action Init:
    accounts = []
    accounts.append(Account(name="Alice"))
    accounts.append(Account(name="Bob"))