Refactoring in eval.  Rename ASTError to EvalError and move it to its own header.  Extract out the ast-input tokenizer function into its own namespace in Token.h / Token.cpp, out of the main eval.cpp.  Other minor cleanups.
Fix tests to accommodate new ID ast output format
Reformatting/cleanups, and note where we'll need to escape single-quotes in token values.
Shell for adding lexer tests