Changed builder protocol to make-node in src/protocol.lisp

* src/protocol.lisp (header): added one-line summary
  (make-comment): removed; replaced by `make-node'
  (make-identifier): likewise
  (make-operator/1): likewise
  (make-operator/2): likewise
  (make-operator/3): likewise
  (make-node): new generic function; used for creation of all nodes in
  new builder protocol
* src/grammar.lisp (define-binary-operator-rule): adapted to changed
  builder protocol
  (define-negation-alternative): likewise
  (defrule unary-expr): likewise
* src/package.lisp (package parser.infix-expressions): removed
  exported symbols make-comment, make-identifier, make-operator/1,
  make-operator/2, make-operator/3; added exported symbol make-node