Add support for reading/writing IELM input history (bug#67000)

Add support for reading/writing IELM input history (bug#67000)

* lisp/ielm.el (inferior-emacs-lisp-mode): Add support for saving input
history to a file.
(ielm--history-file-name): New variable indicating IELM input history
file.
(ielm--exit): Holds a function to call when Emacs is killed to write
out the input history.
(ielm--input-history-writer): Helper function for writing the IELM
input history out to file.

* lisp/comint.el (comint-input-ring-file-name): Improve defcustom tag.