(eldoc-message): Check for 1-arg case, and store string in

(eldoc-message): Check for 1-arg case, and store string in
eldoc-last-message without consing a new string.
Rearrange logic from nested if's into cond's.

(eldoc-print-fnsym-args): Renamed to eldoc-get-fnsym-args-string.
Do not print message; just return string.

(eldoc-get-var-docstring): Renamed from eldoc-print-var-docstring.
Do not print message; just return string.
Cache that string in eldoc-last-data.

(eldoc-last-data): Make into a vector.
(eldoc-get-fnsym-args-string): Use new data form of structure.
(eldoc-get-var-docstring): Here also.

(eldoc-last-data-store): New function.
(eldoc-get-fnsym-args-string): Use it.
(eldoc-get-var-docstring): Here also.

(eldoc-docstring-first-line): Minimize interim string consing.
Call substitute-command-keys on the result.

(eldoc-print-var-docstrings, eldoc-print-docstring,
eldoc-docstring-message): Functions removed.
(eldoc-docstring-format-sym-doc): New function, functionality ripped out of
eldoc-docstring-message.
(eldoc-get-fnsym-args-string, eldoc-get-var-docstring): Use it.