When combined with Flymake mode, which also adds a value to
eldoc-documentation-functions, Python-mode users can now experiment
with different eldoc-documentation-strategy values.

Also, this shoulda allow us to write automatic tests for this
particular Eldoc functionality.

* lisp/progmodes/python.el (inferior-python-mode): Set
coming-preoutput-filter-functions.
(python--shell-output-filter-in-progress)
(python--shell-output-filter-buffer): Rename from python- variant.
(python-shell-output-filter): Rework to support async operation.
(python-eldoc--get-doc-at-point): Rework to support async.
(python-eldoc-function): Use callback.

* lisp/progmodes/cfengine.el (cfengine3-mode): Remove mention to
obsolete eldoc-documentation-function.

* lisp/progmodes/python.el (python-mode): Use with-no-warnings.

* src/fns.c (Fmember, Fassoc, Frassoc): Delegate to the cheaper Fmemq,
Fassq and Frassq for arguments of the appropriate types.
(eq_comparable_value): New function.

* lisp/emacs-lisp/eldoc.el (eldoc-documentation-strategy): Improve
docstring.
(eldoc--make-callback): Improve docstring.
(eldoc--invoke-strategy): New helper function.
(eldoc-print-current-symbol-info): Call eldoc--invoke-strategy.
(eldoc-documentation-functions): Improve docstring.

* lisp/emacs-lisp/eldoc.el (eldoc-documentation-function)
(eldoc-message): Obsolete spec uses eldoc-1.1.0.

* lisp/descr-text.el (describe-char-eldoc): Adjust to new
eldoc-documentation-functions protocol.

* etc/NEWS (Eldoc): Adjust paragraphs.

* lisp/emacs-lisp/eldoc.el (eldoc-prefer-doc-buffer): Adjust
docstring.
(eldoc--enthusiasm-curbing-timer, eldoc-documentation-strategy)
(eldoc-documentation-functions): Adjust docstring.
(eldoc--handle-docs): Adjust comments.
(eldoc--documentation-compose-1): New helper.
(eldoc-documentation-compose)
(eldoc-documentation-compose-eagerly): Use it.
(eldoc-print-current-symbol-info): Adjust comments.