bytes, strings: improve EqualFold fast version for ASCII

The existing implementation only considers the special ASCII
case when the lower character is an upper case letter. This
means that most ASCII comparisons use unicode.SimpleFold even
when it is not necessary.

benchmark                old ns/op     new ns/op     delta
BenchmarkEqualFold-8     450           390           -13.33%

Change-Id: I735ca3c30fc0145c186d2a54f31fd39caab2c3fa
Reviewed-on: https://go-review.googlesource.com/110018


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>