Introduce rr dump --json, dump trace in JSONL, one object per line

[timo]

Combines with some of the other flags:

• --syscallbuf adds a syscalls key
• --mmaps adds a maps key
• --task_events adds a task_events key`
• --recorded-metadata adds a metadata key

Furthermore, also passing --raw will add extra output to some of the above:

• syscallbuf_flush_raw key with the header of a syscallbuf flush
• raw key with the syscall data for each syscall
• raw key with mmap record data to mmaps entries

On top of all that, a --no-regs flag is introduced that skips outputting
register values in the regular, the raw, or the json output.

This also moves write_hex_string from being a static function inside
GdbServerConnection to live in src/util.cc instead.

Here's some example output:

https://gist.github.com/timo/d6dd3b2762d78a9ee468b3cb554fa6b9

Open questions:

• Currently, some functions are duplicated as a _json variant, some functions check flags.json_dump in-line. Is there a preference?
• The current dump function ignores when frame.event().record_extra_regs() is true; do we care about that?
• The initial thing that caused me (well, really dzaima) to look into the code in the first place was that the header of --raw still has three entries in it that correspond to field that are not being output any more. Is that for a separate pull request, or fine to include here?
• The help for the command says that --raw outputs in a different format, and --json makes the same claim, so it's not obvious that they can be combined. The current behaviour of --raw and --json together may want to get a different flag and then --raw could conflict with --json
  • there could instead be a --format=raw/--format=json option instead? though a short flag for json would be nice