Switch org-agenda-get-* defuns to operate on date ranges

[coroa@online.de (Jonas Hörsch)]

hello everyone,

i've just started to use calfw to get a nice calendar-like display of my
agenda. unfortunately i was slighly dismayed at the build-up speed
easily exceeding a few seconds on my not so packed schedule. profiling
into the code i found that the problem mainly is:

there exists only the possibility to retrieve agenda data for one day
day, org-agenda.el's org-agenda-get-day-entries, rather than for a range
of days. this makes it necessary to loop over --get-day-entries to collect
all entries to show in the monthly calendar.

--get-day-entries for its part has to parse (basically regex-search
through) the respective org agenda file completely on each invocation;
effectively done by org-agenda-get-{todos,blocks,sexps,...}.

i imagine it is quite feasible to implement
(defun org-agenda-get-entries (file beg end &rest args) ...)
to sift through FILE but once and return an alist like
((date . (entry1 entry2 ..)) ..)
containing all entries between dates BEG and END, inclusive.

neglecting the extra overhead for sorting entries into the alist, this
promises to speed up typical weekly agenda collection by a factor of 7
and calfw even by a factor of approx. 30, shouldn't it?

before i start a local branch to try myself on this refactoring work,
i'd welcome a few comments, why i should be likely to fail or succeed!

cheers,
jonas