Move interactive `transpose-regions' to Lisp

Define `transpose-regions' in Lisp, because its complex interactive
spec was ungainly in C, and change the C version to non-interactive
`transpose-regions-internal'.  The Lisp function is just a wrapper
around the C function, which still does all the work.

* lisp/simple.el (transpose-regions): New wrapper function, with
  interactive spec taken from old C `transpose-regions'.

* src/editfns.c (Ftranspose_regions): Rename to...
  (Ftranspose_regions_internal): ...here, and remove interactive spec.

Discussion on Emacs Devel:

  From: Karl Fogel
  To: Emacs Development
  Cc: Richard Copley, Charles A. Roelli
  Subject: Re: [Emacs-diffs] master b88e7c8: \
           Make transpose-regions interactive (Bug#30343)
  Date: Fri, 16 Mar 2018 10:23:31 -0500
  Message-ID: <87po44jb7w.fsf@red-bean.com>

  https://lists.gnu.org/archive/html/emacs-devel/2018-03/msg00555.html

2 files changed, 31 insertions, 19 deletions

diff --git a/lisp/simple.el b/lisp/simple.el
index fa93cf87c7a..543fa08233c 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -7014,6 +7014,34 @@ With argument 0, interchanges line point is in with line mark is in."
                        (forward-line arg))))
                   arg))
 
+(defun transpose-regions (startr1 endr1 startr2 endr2 &optional leave-markers)
+  "Transpose region STARTR1 to ENDR1 with STARTR2 to ENDR2.
+The regions should not be overlapping, because the size of the buffer is
+never changed in a transposition.
+
+Optional fifth arg LEAVE-MARKERS, if non-nil, means don't update
+any markers that happen to be located in the regions.
+
+Transposing beyond buffer boundaries is an error.
+
+Interactively, STARTR1 and ENDR1 are point and mark; STARTR2 and ENDR2
+are the last two marks pushed to the mark ring; LEAVE-MARKERS is nil.
+If a prefix argument N is given, STARTR2 and ENDR2 are the two
+successive marks N entries back in the mark ring.  A negative prefix
+argument instead counts forward from the oldest mark in the mark
+ring."
+  (interactive
+   (if (< (length mark-ring) 2)
+       (error "Other region must be marked before transposing two regions")
+     (let* ((num (if current-prefix-arg
+                     (prefix-numeric-value current-prefix-arg)
+                   0))
+            (ring-length (length mark-ring))
+            (eltnum (mod num ring-length))
+            (eltnum2 (mod (1+ num) ring-length)))
+       (list (point) (mark) (elt mark-ring eltnum) (elt mark-ring eltnum2)))))
+  (transpose-regions-internal startr1 endr1 startr2 endr2 leave-markers))
+
 ;; FIXME seems to leave point BEFORE the current object when ARG = 0,
 ;; which seems inconsistent with the ARG /= 0 case.
 ;; FIXME document SPECIAL.
diff --git a/src/editfns.c b/src/editfns.c
index d26319441b3..e24867298e1 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -5112,16 +5112,7 @@ transpose_markers (ptrdiff_t start1, ptrdiff_t end1,
     }
 }
 
-DEFUN ("transpose-regions", Ftranspose_regions, Stranspose_regions, 4, 5,
-       "(if (< (length mark-ring) 2)\
-            (error \"Other region must be marked before transposing two regions\")\
-          (let* ((num (if current-prefix-arg\
-                         (prefix-numeric-value current-prefix-arg)\
-                        0))\
-                 (ring-length (length mark-ring))\
-                 (eltnum (mod num ring-length))\
-                 (eltnum2 (mod (1+ num) ring-length)))\
-            (list (point) (mark) (elt mark-ring eltnum) (elt mark-ring eltnum2))))",
+DEFUN ("transpose-regions-internal", Ftranspose_regions_internal, Stranspose_regions_internal, 4, 5, 0,
        doc: /* Transpose region STARTR1 to ENDR1 with STARTR2 to ENDR2.
 The regions should not be overlapping, because the size of the buffer is
 never changed in a transposition.
@@ -5129,14 +5120,7 @@ never changed in a transposition.
 Optional fifth arg LEAVE-MARKERS, if non-nil, means don't update
 any markers that happen to be located in the regions.
 
-Transposing beyond buffer boundaries is an error.
-
-Interactively, STARTR1 and ENDR1 are point and mark; STARTR2 and ENDR2
-are the last two marks pushed to the mark ring; LEAVE-MARKERS is nil.
-If a prefix argument N is given, STARTR2 and ENDR2 are the two
-successive marks N entries back in the mark ring.  A negative prefix
-argument instead counts forward from the oldest mark in the mark
-ring.  */)
+Transposing beyond buffer boundaries is an error.  */)
   (Lisp_Object startr1, Lisp_Object endr1, Lisp_Object startr2, Lisp_Object endr2, Lisp_Object leave_markers)
 {
   register ptrdiff_t start1, end1, start2, end2;
@@ -5581,5 +5565,5 @@ functions if all the text being accessed has this property.  */);
   defsubr (&Swiden);
   defsubr (&Snarrow_to_region);
   defsubr (&Ssave_restriction);
-  defsubr (&Stranspose_regions);
+  defsubr (&Stranspose_regions_internal);
 }