v1.4.361

Add api_dump_fast_writer to improve write overhead

- ApiDumpFastWriter has the same << syntax as an ostream.
  It gathers pieces in a 512-byte stack buffer, formats numbers
  with std::to_chars, and hands them to the stream's buffer in
  one call. Types it doesn't know go to the real ostream in order.
- the JSON branches of dump_value, dump_start, dump_end, the
  dump_json_* helpers, the array separators and dump_json_UNUSED
  now write through the writer. Text and HTML behaviour is
  unchanged. ArrayElementName no longer uses snprintf.
- 8 new tests, covering writer-versus-ostream equivalence and an
  exact-bytes JSON golden.