Core: limit depth of received JSON to 16

[black-sliver]

What is this fixing or adding?

Limits the maximum depth of a received JSON document to 16.

This applies to both MultiServer and CommonClient.

This means DataStorage default is limited to 14 levels, DataStorage op arg is limited to 13 levels.

How was this tested?

Unit tests.

[GinjaNinja32]

this doesn't handle backslash-escaped quotes, and mismatched backslash-escaped quotes go particularly badly:

{"command": "escaped unclosed \"quote"} => "malformed" despite being valid JSON
{..., {"at_max_depth": "a string with \"quoted [things]\""}, ...} => "too complex" despite not actually being more complex than a version saying "... \"quoted things\""
{..., {"at_max_depth": "\"]]]]\"", "a_complex_object": ..., "whoops": "\"[[[[\""}, ...} => passes despite being more complex than permitted

[black-sliver]

Oh, true. Why did i forget about this? :/ I blame the summer.

[black-sliver]

Some notes on the cython implementation:

• While the python implementation is actually pretty slow, the cython one is pretty fast. For a "real" WebHost, I'd strongly recommend to make sure _speedups are built.
• Uses cpdef rather than def to let the compiler generate an optimal cdef that then gets called from or inlined into the public def. cpdef can be faster than def (and for my test setup it was).
• Uses Py_ssize_t to make sure the integer math can't overflow as long as the limit argument is less than SSIZE_MAX and can't underflow at all because 1 - SSIZE_MAX > SSIZE_MIN. CPython's str length is a Py_ssize_t.