gh-119396: Optimize unicode_repr() (#119617)

gh-119396: Optimize unicode_repr() (#119617)

Use stringlib to specialize unicode_repr() for each string kind
(UCS1, UCS2, UCS4).

Benchmark:

+-------------------------------------+---------+----------------------+
| Benchmark                           | ref     | change2              |
+=====================================+=========+======================+
| repr('abc')                         | 100 ns  | 103 ns: 1.02x slower |
+-------------------------------------+---------+----------------------+
| repr('a' * 100)                     | 369 ns  | 369 ns: 1.00x slower |
+-------------------------------------+---------+----------------------+
| repr(('a' + squote) * 100)          | 1.21 us | 946 ns: 1.27x faster |
+-------------------------------------+---------+----------------------+
| repr(('a' + nl) * 100)              | 1.23 us | 907 ns: 1.36x faster |
+-------------------------------------+---------+----------------------+
| repr(dquote + ('a' + squote) * 100) | 1.08 us | 858 ns: 1.25x faster |
+-------------------------------------+---------+----------------------+
| Geometric mean                      | (ref)   | 1.16x faster         |
+-------------------------------------+---------+----------------------+