External Function Support

[gbracha-f5]

System should support typechecking and execution of external functions.

The mechanism is described in the specification, section 4.2. That is still being refined.
The implementation is in progress. Typechecking works, at least for the simple test case. Refinement expected.
Execution still in progress.

[gbracha-f5]

Base case works. Desired improvements/completions: automatically synthesize validation checks form signatures; deal with conversion of stream valued arguments. Deal with type/shape parameters to external functions.