Add Cons type
Parse and Eval Cons rather than Sexp
Fix Cons.Sexp on nested vectors
Add Map and Len to Cons
Use Value.Cons() typecasting method
Use Map in Sexp
Make Cons.Map handle errors
Any value may be converted to Cons
Fix Cons.String
Fix Eval method
Fix special forms
Remove premature test
Fix Len to depend on Cons() on any Value
Fix Map, handle Nil car
Eval all args to builtins and allow nested lists
Merge branch 'cons'