Improve readability of mathtext internal structures.

This changes the repr of mathtext lists from e.g.
```
[Hlist <70.39 41.78 0.00 0.00> [Hlist <0.00 0.00 0.00 0.00> ] [Hlist <37.62 40.09 0.00 0.00> `V` k1.11] [Hlist <32.76 41.78 0.00 0.00> ` ` `l` k10.34]]
```
to
```
Hlist<w=70.39 h=41.78 d=0.00 s=0.00>[Hlist<w=0.00 h=0.00 d=0.00 s=0.00>[], Hlist<w=37.62 h=40.09 d=0.00 s=0.00>[`V`, k1.11], Hlist<w=32.76 h=41.78 d=0.00 s=0.00>[` `, `l`, k10.34]]
```
These are internal structures that should only ever be seen when
debugging mathtext layout and can be quite deeply nested, so a somewhat
concise notation should is good (e.g., backticks for Chars, k for
Kerns), but still...
- indicate what each of the four numbers correspond to
  (width/height/descent shift),
- move the brackets *after* the class name and metrics, which makes it
  clearer what is info about the list instance itself and what are the
  contents of the instance (in particular, the empty hlist at the
  beginning is now clearer),
- separate list items with commas.

Author: anntzer

lib/matplotlib/_mathtext.py

@@ -1236,11 +1236,11 @@ def __init__(self, elements):
         self.glue_order   = 0    # The order of infinity (0 - 3) for the glue
 
     def __repr__(self):
-        return '[%s <%.02f %.02f %.02f %.02f> %s]' % (
+        return '%s<w=%.02f h=%.02f d=%.02f s=%.02f>[%s]' % (
             super().__repr__(),
             self.width, self.height,
             self.depth, self.shift_amount,
-            ' '.join([repr(x) for x in self.children]))
+            ', '.join([repr(x) for x in self.children]))
 
     @staticmethod
     def _determine_order(totals):