Simpler callback protocol for jsonrpc parameters and results

Simpler callback protocol for jsonrpc parameters and results

Instead of introspecting the :params or :result object to discover if
an object is present, and changing the Elisp function call type
(funcall vs apply) accordingly, alway funcall. It's up to the
application to destructure if it wishes. jrpc-lambda can help with
that and keep the application code simple.

* eglot.el (eglot--on-shutdown): Fix indentation.
(eglot--dispatch): Simplify.
(xref-backend-identifier-completion-table)
(xref-backend-definitions, xref-backend-references)
(xref-backend-apropos, eglot-completion-at-point)
(eglot-eldoc-function, eglot-imenu, eglot--apply-text-edits):
Don't use jrpc-mapply.

* jrpc.el (jrpc--process-receive): Allow only keys defined in
JSONRPC2.0
(jrpc--process-receive): Don't overload function call type based
on remote response.
(jrpc-lambda): Return a unary lambda.
(jrpc-request): Simplify.
(jrpc-mapply): Remove.