jsonutils: support cstring (including as Table key); improve docs

[timotheecour]

• support cstring
• support cstring as Table key
• improve docs wrt Table keys, refs https://forum.nim-lang.org/t/7121#44878

That's exactly why I was wondering. Seems like it should work out of the box which is why I was surprised.

• un-disable a test that now works

future work

• support arbitrary keys in Table using the same trick as in the runnableExamples, there's no possible ambiguity there (maybe needs [TODO] add items(table) to allow using algorithms on a table #9217 or providing a similar iterator byPairs)

[juancarlospaco]

Crossreference cstrutils.

[timotheecour]

Crossreference cstrutils.

not necessarily opposed to it, but where and how ? not obvious how they'd relate