Add EvalStream to support "computing on the code" use cases

Streaming Expression presents interesting opportunities for "computing on the code". The EvalStream will call toExpression() on an underlying stream and then compile and run the resulting expression. This will allow developers to write Streaming Expressions that dynamically build other Streaming Expressions.

This type of behavior could probably be already done in the open() method, but having an EvalStream adds specific support for dynamic generation of Streaming Expressions and should encourage these types of applications.