bug#22038: 25.1.50; Character folding issues with isearch

[Stephen Berman <stephen.berman@gmx.net>]

Issue 1: Please support having multiple characters match a single
string in searches, so that e.g. "ss" can match the German letter "ß".

Issue 2: The current implementation of character folding based on
character decomposition often yields surprising results when searching:
e.g. the search string "f" matches not only "ff" but also "㎙" and "ffl",
but the search strings "m" and "fm" fail to match the former and the
search strings "l" and "fl" fail to match the latter.  I think all
recognizable characters in a composite character should be matchable by
searching.

Issue 3: Character folding does not respect case-folding in searches,
e.g. "f" fails to match "ℱ" and "℻" (with case-folding enabled), whereas
"F" does match them, but fails to match "ff".


In GNU Emacs 25.1.50.1 (x86_64-suse-linux-gnu, GTK+ Version 3.14.15)
 of 2015-11-20
Repository revision: 5c81fd58e32d965c2551663622e084f2800e1e90
Windowing system distributor 'The X.Org Foundation', version 11.0.11601000
System Description:	openSUSE 13.2 (Harlequin) (x86_64)

Configured using:
 'configure 'CFLAGS=-Og -g3''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND DBUS GCONF GSETTINGS NOTIFY
GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix