215 - ignore initial value of out parameters, 220 - implicitly assign out parameters

Fixes #215.
Fixes #220 (well, the `out` parts; will spin the `ref` parts off into another issue).

As discussed in #220, pairing the `out` part of that issue with #215, as releasing #215 by itself would mean that #220 becomes a breaking change.

1. If it's a `ref` parameter, then today it _has_ to be an equality comparison
2. I feel a little dirty with these checks, but `GetArgumentConstraintFromExpression` can't distinguish the ref-ness of the parameter. Eventually that method may change to interpret @patrik-hagne's suggested in-line ref-specfiers, but there was no need to do that today, and I'm afraid it will be significant work.