scripts: Fully connected graph in stack.py, no more recursive folding

This makes -D/--define more useful in stack.py/perf.py/perfbd.py by no
longer hiding undfined children entries.

For example:

  $ ./scripts/stack.py lfs.ci lfs_util.ci -Dfunction=lfsr_mount -t
  function                       frame    limit
  lfsr_mount                        96     2816
  |-> lfsr_fs_gc                    80     2720
  |-> lfsr_mtree_gc                176     2640
  |-> lfsr_mdir_commit             576     2464
  ... snip ...

Now shows all functions in the hot path of lfsr_mount, where before it
would only show functions in the hot path of lfsr_mount that were also
_named_ lfsr_mount.

The previous behavior was technically not wrong... but not very useful
(and confusing).

---

This was actually quite a bit annoying to get working because of the
possibility of function call cycles.

I ended up turning stack.py's result type into a fully connected graph,
which only works because Python has a cycle detector. (Actually this
script is so short-lived we probably wouldn't care if this leaked
memory.)

A nice side effect of this is now all the recursive scripts (stack.py,
perf.py, and perfbd.py) share the same internal result representation
and recursive printing logic, which is probably a good thing.

scripts/perfbd.py

@@ -621,15 +621,6 @@ def fold(Result, results, by=None, defines=[]):
     for name, rs in folding.items():
         folded.append(sum(rs[1:], start=rs[0]))
 
-    # fold recursively
-    folded_ = []
-    for r in folded:
-        folded_.append(r._replace(children=fold(
-            Result, r.children,
-            by=by,
-            defines=defines)))
-    folded = folded_
-
     return folded
 
 def table(Result, results, diff_results=None, *,
@@ -802,7 +793,8 @@ def table(Result, results, diff_results=None, *,
             depth_ = 2
         elif m.isinf(depth_):
             def rec_depth(results_, seen=set()):
-                # rebuild our tables at each layer
+                # build the children table at each layer
+                results_ = fold(Result, results_, by=by)
                 table_ = {
                     ','.join(str(getattr(r, k) or '') for k in by): r
                     for r in results_}
@@ -835,7 +827,8 @@ def table(Result, results, diff_results=None, *,
         if hot:
             def recurse(results_, depth_, seen=set(),
                     prefixes=('', '', '', '')):
-                # rebuild our tables at each layer
+                # build the children table at each layer
+                results_ = fold(Result, results_, by=by)
                 table_ = {
                     ','.join(str(getattr(r, k) or '') for k in by): r
                     for r in results_}
@@ -893,14 +886,14 @@ def table(Result, results, diff_results=None, *,
         else:
             def recurse(results_, depth_, seen=set(),
                     prefixes=('', '', '', '')):
-                # rebuild our tables at each layer
+                # build the children table at each layer
+                results_ = fold(Result, results_, by=by)
                 table_ = {
                     ','.join(str(getattr(r, k) or '') for k in by): r
                     for r in results_}
                 names_ = list(table_.keys())
 
-                # sort again at each layer, keep in mind the numbers are
-                # changing as we descend
+                # sort the children layer
                 names_.sort()
                 if sort:
                     for k, reverse in reversed(sort):
@@ -948,8 +941,7 @@ def table(Result, results, diff_results=None, *,
                              prefixes[2+is_last] + "|   ",
                              prefixes[2+is_last] + "    "))
 
-        # we have enough going on with diffing to make the top layer
-        # a special case
+        # the top layer is a bit of a special case
         for name, line in zip(names, lines[1:-1]):
             print('%-*s  %s' % (
                 widths[0], line[0][0],