improve definitions of various result structures

* add type for %PRODUCTION slot of RESULT structure

* disabled generation of unused copy functions: ERROR-RESULT,
  INACTIVE-RULE, FAILED-PARSE, LEFT-RECURSION-RESULT, RESULT

* make structure slots read-only where possible:
  * INACTIVE-RULE: RULE
  * FAILED-PARSE: EXPRESSION, POSITION, DETAIL
  * LEFT-RECURSION-RESULT: RULE
  * RESULT: POSITION
remove deprecated function and DEFRULE option CONCAT

* Has been deprecated for three years.
mention example-left-recursion.lisp in the manual
add explanation to symbol table example

* The use of :AROUND probably warrants an explanation.
test case-sensitive rules in parse.case-insensitive
update and reorganize TODO file