Add support for retrieving paths to JSON elements

Add support for retrieving paths to JSON elements

Add support for retrieving the path to a JSON element. This can for
instance be useful to retrieve paths in deeply nested JSON
structures.

* lisp/json.el (json-pre-element-read-function)
(json-post-element-read-function): New variables to hold pre- and post
read callback functions for `json-read-array' and `json-read-object'.
(json--path): New variable used internally by `json-path-to-position'.
(json--record-path, json--check-position): New functions used
internally by `json-path-to-position'.
(json-path-to-position): New function for retrieving the path to a
JSON element at a given position.
(json-read-object, json-read-array): Call
`json-pre-element-read-function' and `json-post-element-read-function'
when set.

* test/automated/json-tests.el (test-json-path-to-position-with-objects)
(test-json-path-to-position-with-arrays)
(test-json-path-to-position-no-match): New tests for
`json-path-to-position'.