Simplify some function calling infrastructure

Simplify some function calling infrastructure

eglot--mapply is a confusing abstraction. Hide some of that confusion
behind eglot--lambda. More stably dispatch server notifications and
requests without introspecting their contents.

* eglot.el (eglot--process-receive): Simplify.
(eglot--async-request): Improve doc.
(eglot--request): Simplify.
(eglot--mapply): Remove.
(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 eglot--mapply, use normal mapcar/mapc.