gh-119396: Optimize unicode_repr()

[vstinner]

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         |
+-------------------------------------+---------+----------------------+

• Issue: Optimize _PyUnicodeWriter implementation #119396

[vstinner]

Benchmark:

import pyperf
runner = pyperf.Runner()
squote = "'"
dquote = '"'
nl = '\n'
runner.bench_func("repr('abc')", repr, 'abc')
runner.bench_func("repr('a' * 100)", repr, 'a' * 100)
runner.bench_func("repr(('a' + squote) * 100)", repr, ('a' + squote) * 100)
runner.bench_func("repr(('a' + nl) * 100)", repr, ('a' + nl) * 100)
runner.bench_func("repr(dquote + ('a' + squote) * 100)", repr, dquote + ('a' + squote) * 100)

[vstinner]

cc @serhiy-storchaka

[vstinner]

This is a first step. The second step will be to avoid a temporary string in PyUnicode_FromFormat("%R", str_obj).

[vstinner]

This is a first step. The second step will be to avoid a temporary string in PyUnicode_FromFormat("%R", str_obj).

I implemented the second step locally. Sadly, it's slower! Not faster. IMO the first step (making the code faster) is still worth it :-)