Prune most-positive-fixnum from Lisp source

Prune most-positive-fixnum from Lisp source

I looked through all instances of most-negative-fixnum
and most-positive-fixnum in the Lisp source code, and
when it was easy I removed assumptions that integers
fit in fixnums.  The remaining instances are either
nontrivial to fix, or are inherent to the algorithm.
* lisp/arc-mode.el (archive-l-e): Do not convert to float,
since we have bignums now.  All uses changed.
* lisp/calc/calc.el (math-bignum):
Don’t special-case most-negative-fixnum.
* lisp/calendar/parse-time.el (parse-time-string):
* lisp/emacs-lisp/edebug.el (edebug-read-special):
* lisp/emacs-lisp/package.el (package--remove-hidden):
* lisp/gnus/nnfolder.el (nnfolder-read-folder):
* lisp/international/mule-util.el (filepos-to-bufferpos--dos):
* lisp/menu-bar.el (menu-bar-update-buffers):
* lisp/net/rcirc.el (rcirc-handler-317):
* lisp/org/org-agenda.el (org-cmp-ts):
* lisp/window.el (window--resize-child-windows):
Avoid arbitrary limit to most-positive-fixnum or to
most-negative-fixnum.
* lisp/calendar/time-date.el (days-to-time):
* lisp/erc/erc-dcc.el (erc-unpack-int):
Don’t worry about integer overflow.
* lisp/cedet/semantic/wisent/comp.el (wisent-BITS-PER-WORD):
* lisp/gnus/message.el (message-unique-id):
* lisp/org/org-footnote.el (org-footnote-new):
Simplify.
* lisp/erc/erc-dcc.el (erc-most-positive-int-bytes)
(erc-most-positive-int-msb): Remove; no longer needed.
* lisp/net/imap.el (imap-string-to-integer): Remove; unused.
* lisp/org/org-element.el (org-element--cache-generate-key):
Document fixnum limitation.