Introduce EvaluationContext for statement and expression evaluation

[lahma]

Needed in generators and also other benefits, took out of generators as it's getting big enough PR without this.

• state information can flow from engine entry inside single context, internal EvaluationContext
• now _engine * field is no longer required in statement or expression base classes (less memory usage)
• should also open scenarios like per call constraints instead of global ones fixed in engine, like
  • engine.Evaluate("my script", perCallOptions => DoDonfigure(perCallOptions))
• this should also separate better evaluation time information and engine infrastructure, maybe at some point Engine doesn't need to be passed inside the context at all

Biggest API change is that I've obsoleted JsValue.Invoke and now requiring engine parameter. It's a bit scary invoke facility outside of engine anyway.

[lahma]

Also extracted Completion handling from the old generators PR.

Now expression evaluation produces ExpressionResult which hold object result, which also allows Reference, old logic had object return type all around which can cause boxing of Completion, now statements return Completion and there's conversion to between ExpressionResult and Completion. Statements never return references so Completion's JsValue is always valid.

I was able to squash couple of bugs also related to array pattern handling and simplified DeclarativeEnvironmentRecord a bit

[sebastienros]

Can you summarize the breaking changes in the API ? Apart from the Invoke change.