perf(sourcemap): speed up encoding mappings

[overlookmotel]

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[overlookmotel]

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• perf(sourcemap): speed up encoding mappings #4492 👈
• fix(sourcemap): fix pre-calculation of required segments for building JSON #4490
• perf(sourcemap): reduce memory copies encoding JSON #4489
• refactor(sourcemap): push_list method for building JSON #4486
• main

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[codspeed-hq]

CodSpeed Performance Report

Merging #4492 will not alter performance

_{Comparing 07-26-perf_sourcemap_speed_up_encoding_mappings (a02c47c) with main (5db7bed)}

Summary

✅ 32 untouched benchmarks

[overlookmotel]

No appreciable change on benchmarks, so it's not worth the unsafe code. Have pulled out aligning the base64 lookup table into a separate PR #4535.