fix(CharsetToUTF8): use strings.Builder (#154)

Co-authored-by: akondratev <artem.kondratev@goods.ru>

internal/cp/charset.go

@@ -1,5 +1,9 @@
 package cp
 
+import (
+	"strings"
+)
+
 type charsetMap struct {
 	sb [256]rune    // single byte runes, -1 for a double byte character lead byte
 	db map[int]rune // double byte runes
@@ -91,7 +95,9 @@ func CharsetToUTF8(col Collation, s []byte) string {
 	if cm == nil {
 		return string(s)
 	}
-	buf := make([]rune, 0, len(s))
+
+	buf := strings.Builder{}
+	buf.Grow(len(s))
 	for i := 0; i < len(s); i++ {
 		ch := cm.sb[s[i]]
 		if ch == -1 {
@@ -107,7 +113,7 @@ func CharsetToUTF8(col Collation, s []byte) string {
 				}
 			}
 		}
-		buf = append(buf, ch)
+		buf.WriteRune(ch)
 	}
-	return string(buf)
+	return buf.String()
 }