Show HN: simdown – use SIMD to parse Markdown at ~1 GB/s

The SIMD path currently needs arm64 (NEON) and GOEXPERIMENT=simd. I've only benchmarked on an Apple M4, but it should help on any arm64 chip (Graviton, Ampere, etc.). Elsewhere it falls back to a scalar path that does ~300 MB/s, still ahead of cmark and goldmark.

If you know of a faster CommonMark parser, please tell me and I'll add it to the benchmarks.

Under the hood, it borrows a lot from this paper on using simd for json parsing: https://arxiv.org/abs/1902.08318