Import or otherwise use testsuite from "Parsing JSON is a Minefield 💣"

[fingolfin]

See https://seriot.ch/projects/parsing_json.html

I did not yet look, but it seems that it should be possible to hook up the json package with the test runner of that project... we might not be able to pass all tests, esp. those related to unicode / UTF-16, but I wonder about e.g. crashes.

[ChrisJefferson]

I did a quick and dirty hook-up, and fixes a couple of things that weren't getting accepted, and some things that shouldn't have been parsed (like [-]). Now everything is accepted that should be, but we accept a bunch of extra things.

There is a bunch of number formats we technically shouldn't be parsing (like 012 we parse as 12 rather than ignoring as possible octal, and 2.2e4), because I pass them through to GAP early, and GAP happily parses them. I wouldn't mind if someone wanted to make the parser stricter, but I'm not bothered.

The only thing we now don't parse is UTF-16, which I certainly don't care about.