Vectorize basic_string::find

[AlexGuteniev]

Resolves #5036

⚠️ Need to decide whether this CRT replacing vectorization is needed ⚠️

🏁 Race against the runtime!

basic_string::find is already vectorized for 8 and 16 bit characters via calling memchr and wmemchr

memchr shows good results for smaller positions, but if the distance from the beginning to the result is long the memchr results are noticeably worse. There's strong indication that such a results are due to not using AVX in memchr.

wmemchr is not fast at all. It was promised to be fast in a new CRT in #4873 though.

For larger characters the vectorization is novel, there's no CRT implementation, but these characters are rare anyway.

⏱️ Benchmark results

Benchmark	main	this
bm<char, not_highly_aligned_allocator, Op::StringFind>/8021/3056	89.6 ns	49.3 ns
bm<char, not_highly_aligned_allocator, Op::StringFind>/63/62	11.3 ns	3.70 ns
bm<char, not_highly_aligned_allocator, Op::StringFind>/31/30	9.90 ns	8.70 ns
bm<char, not_highly_aligned_allocator, Op::StringFind>/15/14	4.94 ns	7.10 ns
bm<char, not_highly_aligned_allocator, Op::StringFind>/7/6	2.83 ns	3.49 ns
bm<char, highly_aligned_allocator, Op::StringFind>/8021/3056	78.0 ns	48.0 ns
bm<char, highly_aligned_allocator, Op::StringFind>/63/62	12.4 ns	4.11 ns
bm<char, highly_aligned_allocator, Op::StringFind>/31/30	10.2 ns	7.85 ns
bm<char, highly_aligned_allocator, Op::StringFind>/15/14	5.18 ns	7.42 ns
bm<char, highly_aligned_allocator, Op::StringFind>/7/6	2.76 ns	3.42 ns
bm<wchar_t, not_highly_aligned_allocator, Op::StringFind>/8021/3056	754 ns	85.3 ns
bm<wchar_t, not_highly_aligned_allocator, Op::StringFind>/63/62	31.1 ns	3.84 ns
bm<wchar_t, not_highly_aligned_allocator, Op::StringFind>/31/30	16.2 ns	2.92 ns
bm<wchar_t, not_highly_aligned_allocator, Op::StringFind>/15/14	4.65 ns	3.66 ns
bm<wchar_t, not_highly_aligned_allocator, Op::StringFind>/7/6	2.45 ns	2.93 ns
bm<char32_t, not_highly_aligned_allocator, Op::StringFind>/8021/3056	749 ns	159 ns
bm<char32_t, not_highly_aligned_allocator, Op::StringFind>/63/62	18.5 ns	5.25 ns
bm<char32_t, not_highly_aligned_allocator, Op::StringFind>/31/30	9.60 ns	3.55 ns
bm<char32_t, not_highly_aligned_allocator, Op::StringFind>/15/14	6.42 ns	2.91 ns
bm<char32_t, not_highly_aligned_allocator, Op::StringFind>/7/6	2.80 ns	2.95 ns