add recursion depth limit

[nigoroll]

for validating untrusted inputs, we need to limit the recursion depth in order to avoid running into stack overflows.

We also introduce a new extensible API interface: json_walk_args() takes additional arguments in the form of a struct which can be extended easily and in a backwards-compatible manner.

[nigoroll]

context: https://code.uplex.de/uplex-varnish/libvmod-frozen

[nigoroll]

updated via force push

[nigoroll]

I have submitted https://cesanta.com/cla.html and rebased onto c6a8e52b9af9e8be34737a2bd58f2398b784df52

[nigoroll]

Dear maintainers, could you please let me know if this project is actually taking outside contributions?

[cpq]

@nigoroll Merged with thanks