Step one of optimizations for StrCaseCmp:

Step one of optimizations for StrCaseCmp:

First of all, do a char-by-char walk through both buffers until we get
to a non-ascii character, or a difference between the strings.  This
prefix can be directly compared without needing to call into iconv.
This should be much faster for strings that are either all ascii, or
differ near the start.
(This used to be commit f7f692b2db4dd513068d6d8fed2792186933ddda)