Implement variable-expression definitions.
Introduce forms, programs.
Correct previous mistake, body is a list of expressions not forms.
Another correction, body should be a list of definitions and a list of expressions.
let and letrec should also use Body instead of Expression list.
Simplify definitions.
Implement let and letrec, partially (properly) implement let-syntax and letrec-syntax.
Merge "pair" with "list". Remove some unneeded parser junk. Refine identifier grammar to match r5rs