Added a keyword test to make-ptree (for string ptree IDs).

[c-kloukinas]

Now it's possible to create ptrees that test compare their identities with any of eq/eql/equal/equalp (that the underlying nodes hash map slot accepts), so we can identify them with strings (or so we can use eq with symbols).
Default test is still eql, as it was originally.

Run locally all tests on sbcl/abcl/ccl/ecl - all passed.

Side note: ecl produces a number of warnings, mainly about lists being used instead of vectors, like:

;;; Style warning:
;;;   in file cognate-test.lisp, position 46050
;;;   at (FULL-TEST FUNCTION-DESIGNATORS-TEST ...)
;;;   ! The argument 1 of function FILL-POINTER-SET has type
;;; LIST
;;; instead of expected
;;; VECTOR
;;; Finished compiling /home/christos/src/lparallel-chris/test/cognate-test.lisp

And some other warnings:

;;; Warning:
;;;   in file defpun-test.lisp, position 1685
;;;   at (DEFINE-PLET-TEST DEFPUN-BASIC-TEST ...)
;;;   ! Unsupported CONS type LPARALLEL.KERNEL::TASK. Replacing it with T.
;;; Warning:
;;;   in file defpun-test.lisp, position 1685
;;;   at (DEFINE-PLET-TEST DEFPUN-BASIC-TEST ...)
;;;   ! Unsupported CONS type LPARALLEL.KERNEL::TASK. Replacing it with T.
;;; Finished compiling /home/christos/src/lparallel-chris/test/defpun-test.lisp.

[c-kloukinas]

Looks like the system is currently maintained elsewhere - moved the PR there