feat(data_structures): introduce AsciiChar

[overlookmotel]

Introduce AsciiChar. Copied from std::ascii::Char.

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[overlookmotel]

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• perf(codegen): use AsciiChar for statically unknowable characters #6538
• feat(data_structures): introduce AsciiChar #6537 👈
• refactor(codegen)!: Codegen::into_source_text consume Codegen #6539
• docs(codegen): fix example for CodeBuffer::print_ascii_bytes #6535
• perf(codegen): check last char with byte methods #6509
• perf(codegen): optimize CodeBuffer::print_ascii_byte #6516
• main

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[codspeed-hq]

CodSpeed Performance Report

Merging #6537 will not alter performance

_{Comparing 10-13-feat_data_structures_introduce_asciichar_ (50e6c68) with main (7e909a7)}

Summary

✅ 29 untouched benchmarks

[Boshen]

Block merge. Needs discussion.

[overlookmotel]

We discussed. Have closed #6538. AsciiChar is a useful abstraction which could be potentially used in lexer etc, but right now we have no use for it, so closing for now. We can can come back to it later if we find uses for it.