true and false need to be removed from Parser

[jcubic]

This will be a breaking change, but because of true and false are constant values like #t and #f this code doesn't work:

(define-syntax foo
  (syntax-rules ()
    ((_ true)
     '(list true))))

(let ((x #t))
  (print (foo x)))

the code is based on aif macro from SRFI-139.

To be compatible with Scheme, those constants need to be removed from Parser and added as aliases.

It will be a breaching change because of code like this:

Old code

'(true)
;; ==> (#t)

New Code:

'(true)
;; ==> (true)

true and false will be normal symbols.

[jcubic]

There are still null and undefined I'm not sure if they should also be removed from the parser.