scripts: csv.py: Simplified -i/--enumerate to alias -bi -Fi=enumerate()

Now that we have the enumerate expr, -i/--enumerate can be implemented
entirely during expr eval:

- -i/--enumerate        => -bi -Fi=enumerate()
- -I/--hidden-enumerate => -Bi -Fi=enumerate()

Instead of internally reimplementing the same behavior.

This is what our help text implies, so might as well put our money where
our mouth is. And the less special internals we have, the better.

I considered removing -i/-I completely, but it's quite a convenient flag
when debugging csv.py expressions.

scripts/csv.py

@@ -1713,7 +1713,6 @@ def compile(fields_, results,
                 **{'_notes': notes} if notes is not None else {}))
 
 def homogenize(Result, results, *,
-        enumerates=None,
         defines=[],
         depth=1,
         depth_=0,
@@ -1737,17 +1736,12 @@ def homogenize(Result, results, *,
         # append a result
         results_.append(Result(
                 **(r
-                    # enumerate?
-                    | ({e: len(results_) for e in enumerates}
-                        if enumerates is not None
-                        else {})
                     # keep track of depth?
                     | ({Result._z: depth_} if hasattr(Result, '_z') else {})
                     # recurse?
                     | ({Result._children: homogenize(
                             Result, r[Result._children],
                             # only filter defines at the top level!
-                            enumerates=enumerates,
                             depth=depth-1,
                             depth_=depth_+1)}
                         if hasattr(Result, '_children')
@@ -2421,19 +2415,14 @@ def main(csv_paths, *,
                 notes=notes,
                 **args)
 
-    # separate out enumerates/mods/exprs
+    # separate out mods/exprs
     #
-    # enumerate enumerates: -ia
     # by supports mods: -ba=%(b)s
     # fields/sort/etc supports exprs: -fa=b+c
     #
-    enumerates = [k
-            for (k, v), hidden in (by or [])
-                if v == enumerate]
     mods = [(k, v)
             for k, v in it.chain(
-                ((k, v) for (k, v), hidden in (by or [])
+                ((k, v) for (k, v), hidden in (by or [])))
-                    if v != enumerate))
             if v is not None]
     exprs = [(k, v)
             for k, v in it.chain(
@@ -2515,7 +2504,6 @@ def main(csv_paths, *,
 
     # homogenize
     results = homogenize(Result, results,
-            enumerates=enumerates,
             defines=defines,
             depth=depth)
 
@@ -2551,7 +2539,6 @@ def main(csv_paths, *,
 
         # homogenize
         diff_results = homogenize(Result, diff_results,
-                enumerates=enumerates,
                 defines=defines,
                 depth=depth)
 
@@ -2638,29 +2625,35 @@ if __name__ == "__main__":
             '-a', '--all',
             action='store_true',
             help="Show all, not just the ones that changed.")
+    class AppendEnumerate(argparse.Action):
+        def __call__(self, parser, namespace, value, option):
+            if namespace.by is None:
+                namespace.by = []
+            if namespace.fields is None:
+                namespace.fields = []
+            namespace.by.append(((value, None), option in {
+                    '-I', '--hidden-enumerate'}))
+            namespace.fields.append(((value, CsvExpr('enumerate()')), True))
+    parser.add_argument(
+            '-i', '--enumerate',
+            action=AppendEnumerate,
+            nargs='?',
+            const='i',
+            help="Enumerate results with this field, equivalent to "
+                " -bi -Fi=enumerate(). This will prevent result folding.")
+    parser.add_argument(
+            '-I', '--hidden-enumerate',
+            action=AppendEnumerate,
+            nargs='?',
+            const='i',
+            help="Like -i/--enumerate, but hidden from the table renderer, "
+                "and doesn't affect -b/--by defaults.")
     class AppendBy(argparse.Action):
         def __call__(self, parser, namespace, value, option):
             if namespace.by is None:
                 namespace.by = []
             namespace.by.append((value, option in {
-                    '-B', '--hidden-by',
+                    '-B', '--hidden-by'}))
-                    '-I', '--hidden-enumerate'}))
-    parser.add_argument(
-            '-i', '--enumerate',
-            action=AppendBy,
-            nargs='?',
-            type=lambda x: (x, enumerate),
-            const=('i', enumerate),
-            help="Enumerate results with this field, equivalent to "
-                " -bi -Fi=enumerate(). This will prevent result folding.")
-    parser.add_argument(
-            '-I', '--hidden-enumerate',
-            action=AppendBy,
-            nargs='?',
-            type=lambda x: (x, enumerate),
-            const=('i', enumerate),
-            help="Like -i/--enumerate, but hidden from the table renderer, "
-                "and doesn't affect -b/--by defaults.")
     parser.add_argument(
             '-b', '--by',
             action=AppendBy,