[Grace-core] Grace interpreter

[kim at cs.pomona.edu]

My Grace interpreter (built to extend Michael's minigrace front-end is
available in the repository in KimGraceInterp.  The interpreter is still
missing important features like classes and support for booleans and
numbers as objects, but the core is there.  Comments are welcome.  In
particular, let me know if you see any problems as it has not been
thoroughly tested on anything other than very simple examples.

A few technical notes:  The interpreter is based on the Visitor Pattern. 
I added (a slightly modified) visitor method to all of the nodes in the
AST.  I hope to also add a type checker using the same mechanism.

KIm