(nth 2) is probably a better alternative to caddr, and in Emacs 26.1
we can pass 0 as :service to automatically find an available port.
* lisp/jsonrpc.el (jsonrpc--call-deferred): Use cl-caddr.
* test/lisp/jsonrpc-tests.el
(jsonrpc--call-with-emacsrpc-fixture): Pass 0 as :service to when
making the listen server.
* jsonrpc.el (Package-Requires): Require Emacs 25.1
(jsonrpc-lambda): Use cl-gensym.
(jsonrpc--call-deferred): Caddr doesn't exist in
emacs 25.1.
* jsonrpc-tests.el
(jsonrpc--call-with-emacsrpc-fixture): New function.
(jsonrpc--with-emacsrpc-fixture): Use it.
(deferred-action-complex-tests): Adjust test for Emacs 25.1
* doc/lispref/text.texi (Text): Add JSONRPC.
(JSONRPC): New node.
* etc/NEWS (New Modes and Packages in Emacs 27.1): Mention jsonrpc.el
* lisp/jsonrpc.el: New file.
* test/lisp/jsonrpc-tests.el: New file.