implement missing core special forms (letrec, letrec*, let-values, etc.)

[kyleterry]

Several key Scheme special forms are currently missing from the evaluator:

• letrec and letrec*: Necessary for mutually recursive local procedures.
• let-values and let*-values: For multiple-value bindings.
• parameterize and make-parameter: For dynamically-scoped parameters.
• cond-expand: For conditional compilation/evaluation.
• include and include-ci: For including external files.