Using the libjansson JSON parser gives us a modest boost in speed.
It's not as significant a speedup as it is for LSP clients since
our jSON payloads are smaller and less frequent -- but we might as
well use it.
* gptel.el (gptel--json-read, gptel--json-encode,
gptel--url-get-response, gptel--parse-response): Define macros to
use the libjansson-supported `json-parse-buffer` and
`json-serialize`. Replace use of `json-encode` and `json-read`
appropriately.
* gptel-openai.el: (gptel-curl--parse-stream) : Use
`gptel--json-read` instead of `json-read`.
* gptel-ollama.el (gptel-curl--parse-stream): Use
`gptel--json-read` instead of `json-read`.
* gptel-gemini.el (gptel-curl--parse-stream): Use
`gptel--json-read` instead of `json-read`.
* gptel-curl.el (gptel-curl--get-args, gptel-curl--get-response,
gptel-curl--log-response, gptel-curl--stream-cleanup,
gptel-curl--parse-response): Use `gptel--json-read` and
`gptel--json-encode` in place of the json.el versions.
* gptel-anthropic.el (gptel-curl--parse-stream): Use
`gptel--json-read` instead of `json-read`.
* test/gptel-org-test.el: Use `gptel--json-read`.
