Fix JSON Pointer array indices and index-like object keys

[jg-rp]

We currently support JSON pointers with negative array indices, but the JSON Pointer specification (RFC 6901) does not allow negative array indexes. See #115.

This PR changes JSONPointer.min_int_index to be 0, causing a JSONPointerIndexError when resolving a negative index against an array-like value.

JSONPointer.min_int_index can be changed to a suitably negative integer if you need to support negative indices in your JSON pointers.

New default behavior

from jsonpath import JSONPointer

data = {"some": {"thing": [1, 2, 3]}}
pointer = JSONPointer("/some/thing/-2")
pointer.resolve(data)  # jsonpath.exceptions.JSONPointerIndexError: pointer index error: array indices must be between 0 and 9007199254740991

Change the minimum allowed index

from jsonpath import JSONPointer

JSONPointer.min_int_index = -(2**53) + 1

data = {"some": {"thing": [1, 2, 3]}}
pointer = JSONPointer("/some/thing/-2")
print(pointer.resolve(data))  # 2

We've also refactored some JSONPointer internals so that an index-like pointer token can be resolved against an object value.

data = {"foo": {"-1": "bar"}}
pointer = JSONPointer("/foo/-1")
assert pointer.resolve(data) == "bar"

Because of this, min and max int indices are now checked at pointer resolution time because we need data to know if we're resolving against an array value or an object value.

[jg-rp]

Another option is to change Pointer.min_int_index to be 0 by default. That way users can choose to support negative indices if they wish.