fix: optimize isASCII, remove rune slice casts

name               old time/op    new time/op    delta
IsASCII/Len=1-12     5.60ns ± 1%    3.28ns ± 0%  -41.37%  (p=0.000 n=9+8)
IsASCII/Len=8-12     18.8ns ± 2%     8.8ns ± 0%  -53.29%  (p=0.000 n=9+10)
IsASCII/Len=16-12    27.2ns ± 2%    16.9ns ± 3%  -37.76%  (p=0.000 n=10+9)
IsASCII/Len=32-12    57.3ns ±14%    30.8ns ± 2%  -46.26%  (p=0.000 n=10+10)

ascii.go

@@ -1,16 +1,19 @@
 package asciicheck
 
-import "unicode"
+import (
+	"unicode"
+	"unicode/utf8"
+)
 
 func isASCII(s string) (rune, bool) {
 	if len(s) == 1 {
-		return []rune(s)[0], s[0] <= unicode.MaxASCII
+		r, size := utf8.DecodeRuneInString(s)
+		return r, size < 2
 	}
 
-	r := []rune(s)
-	for i := 0; i < len(s); i++ {
-		if r[i] > unicode.MaxASCII {
-			return r[i], false
+	for _, r := range s {
+		if r > unicode.MaxASCII {
+			return r, false
 		}
 	}