scripts: csv.py: Added explicit z field, reusing -Z/--children

In an effort to move away from magic usage of -i/--enumerate, this adds
an explicit z field for differentiating -r/--hot results (and for normal
recursive results).

Instead of trying to think of a new flag to control this, this just
piggybacks on -Z/--children, which now accepts a tuple:

- ./scripts/csv.py -z3 -Z
- ./scripts/csv.py -z3 -Zchildren
- ./scripts/csv.py -z3 -Zz,children

The only tricky bit was needing to insert z in front of the by fields,
otherwise it was mostly a simplification from the enumerate mess.

Another positive side-effect: -r/--hot (and -z/--depth) now implies
-Zz,children, removing the annoying/confusing behavior of hotify folding
results by default.

scripts/perf.py

@@ -167,6 +167,7 @@ class PerfResult(co.namedtuple('PerfResult', [
             'cycles': CsvInt,
             'bmisses': CsvInt, 'branches': CsvInt,
             'cmisses': CsvInt, 'caches': CsvInt}
+    _z = 'z'
     _children = 'children'
 
     __slots__ = ()
@@ -913,14 +914,14 @@ def fold(Result, results, *,
     return folded
 
 def hotify(Result, results, *,
-        enumerates=None,
         depth=1,
         hot=None,
         **_):
-    # note! hotifying risks confusion if you don't enumerate/have a
-    # z field, since it will allow folding across recursive boundaries
+    # note! hotifying risks confusion if you don't have a z field, since
+    # it will allow folding across recursive boundaries
 
     # hotify only makes sense for recursive results
+    assert hasattr(Result, '_z')
     assert hasattr(Result, '_children')
 
     results_ = []
@@ -942,12 +943,8 @@ def hotify(Result, results, *,
                                 for k_ in ([k] if k else Result._sort)))
                         for k, reverse in it.chain(hot, [(None, False)])))
 
-            hot_.append(r._replace(**(
-                    # enumerate?
-                    ({e: len(hot_) for e in enumerates}
-                            if enumerates is not None
-                            else {})
-                        | {Result._children: []})))
+            # flatten, dropping children
+            hot_.append(r._replace(**{Result._children: []}))
 
             # recurse?
             if depth_ > 1: