add validation error type, fix tests, add number tests

chars.go

@@ -1,20 +1,18 @@
 package validate
 
 import (
-	"errors"
 	"strings"
 )
 
 // Validation error.
 var (
-	ErrDisallowedChars = errors.New("contains disallowed characters")
+	ErrDisallowedChars = NewError("contains disallowed characters")
 )
 
 // Chars validates whether a string contains only allowed characters.
 func Chars(allow string) func(string) error {
 	return func(value string) error {
-		rs := []rune(value)
-		for _, r := range rs {
+		for _, r := range value {
 			if !strings.ContainsRune(allow, r) {
 				return ErrDisallowedChars
 			}