Attempt at extracting an implementation-neutral but extensible VM.

The first file is a copy of the AST definitions in the SICL Compiler
subdirectory.  Next, we plan to separate general code form specific
code, and document how to make extensions.
Changed the name of the package.
Removed comment that is no longer applicable.
Extracted graphviz drawing code to a separate component.
Removed SICL-specific AST classes.