Asp should handle getting a function's source code

[shoaibkamil]

Currently, each specializer is responsible for obtaining source code of a function if necessary. Asp should have a method to do this (perhaps asp.get_source(foo) or some other variant). Should have meaningful errors thrown if something goes wrong.

For a starting point, see how the stencil specializer does it in stencil_kernel.py.

Est. time: O(hours).