Edited Feature: Code First: Support function imports [818]

**If we don't end up supporting Function Imports on DbContext in EF6 then we should add ExecuteFunctionAsync to ObjectContext**

Enable function imports in Code First.

High level aspects:

- DbModelBuilder FI design
- DbContext call usage patterns
- Update serializers
- Update validators
- Entity results
- Scalar results
- Complex type results
- Void functions
- TVFs
- Multiple result-sets
- InOut/Out parameters