wc: optimize processing of ASCII in multi byte locales

wc: optimize processing of ASCII in multi byte locales

===== Benchmark setup (on GNU/Linux) ====
$ yes áááááááááááááááááááá | head -n100000 > mbc.txt
$ yes 12345678901234567890 | head -n100000 > num.txt

===== Before ====
$ time src/wc -Lm < mbc.txt
real    0m0.186s
$ time src/wc -m < mbc.txt
real    0m0.186s
$ time src/wc -Lm < num.txt
real    0m0.055s
$ time src/wc -m < num.txt
real    0m0.056s

==== After ====
$ time src/wc -Lm < mbc.txt
real    0m0.196s
$ time src/wc -m < mbc.txt
real    0m0.173s
$ time src/wc -Lm < num.txt
real    0m0.031s
$ time src/wc -m < num.txt
real    0m0.028s

* src/wc.c (wc): Only call wide variant functions like
iswprint() and wcwidth() for non is_basic() characters.
I.E. non ISO C "basic character set" characters.
This is especially significant on OSX where wcwidth()
is very expensive (about 10x in tests).
* NEWS: Mention the improvement.
Suggested by Eric Fischer.