Compare json_view with yyjson, simdjson, and Boost.JSON

tests/benchmarks/json_view/ holds a comparison with other
libraries, answering the question users ask when they pick one.
It is not built by CMake and not run by CI.

- bench_view.cpp: parse, traverse, select and dump of twitter,
  citm_catalog, canada, jeopardy, a tweet and an RPC request,
  against json_view, yyjson, simdjson, Boost.JSON and json::parse,
  all agreeing and run interleaved.
- bench_corpus.cpp: parse, traverse and dump of any file list.
- bench_edit.cpp: parses, edits through each library's own API,
  and serializes; all outputs must describe the same value.
- compare.py: builds both programs with system or pinned,
  SHA-256-checked downloads, and writes results with the commit,
  CPU, OS, compiler and library versions.
- README.md and a workflow that runs on demand or via a PR label.

Fairness fixes folded in: each engine runs once untimed before
each timed round, so the next one no longer pays for the
previous one's cleanup; dump is also compared with source numbers
against yyjson's raw-number mode; a "simdjson DOM (fresh)" column
and a reused-document column for json_view were added;
compare.py's paths are absolute; downloads use a .part file and a
failed checksum removes the archive.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/benchmarks/json_view/.gitignore

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+build/