Eval top block

Single top block for Eval named display. This block takes any type (described as promiscuous in a blockly-core comment).
![image](https://cloud.githubusercontent.com/assets/1767466/4910876/7a32fe8e-6485-11e4-9e8b-b2139be4272c.png)

Cleaned up Calc/Eval code some to 
(a) Have a single code generation path
(b) Have a single point where we transfer the object we've created from the generated code to the app code. This instead of using lastExpression/lastObject.