Lazily initialize postcode regexes to speed up init()

Using `GODEBUG=inittrace=1` I found a relatively constant
10ms init caused by this function.

baked_in.go

@@ -1364,6 +1364,7 @@ func isPostcodeByIso3166Alpha2(fl FieldLevel) bool {
 	field := fl.Field()
 	param := fl.Param()
 
+	postcodeRegexInit.Do(initPostcodes)
 	reg, found := postCodeRegexDict[param]
 	if !found {
 		return false

postcode_regexes.go

@@ -1,6 +1,9 @@
 package validator
 
-import "regexp"
+import (
+	"regexp"
+	"sync"
+)
 
 var postCodePatternDict = map[string]string{
 	"GB": `^GIR[ ]?0AA|((AB|AL|B|BA|BB|BD|BH|BL|BN|BR|BS|BT|CA|CB|CF|CH|CM|CO|CR|CT|CV|CW|DA|DD|DE|DG|DH|DL|DN|DT|DY|E|EC|EH|EN|EX|FK|FY|G|GL|GY|GU|HA|HD|HG|HP|HR|HS|HU|HX|IG|IM|IP|IV|JE|KA|KT|KW|KY|L|LA|LD|LE|LL|LN|LS|LU|M|ME|MK|ML|N|NE|NG|NN|NP|NR|NW|OL|OX|PA|PE|PH|PL|PO|PR|RG|RH|RM|S|SA|SE|SG|SK|SL|SM|SN|SO|SP|SR|SS|ST|SW|SY|TA|TD|TF|TN|TQ|TR|TS|TW|UB|W|WA|WC|WD|WF|WN|WR|WS|WV|YO|ZE)(\d[\dA-Z]?[ ]?\d[ABD-HJLN-UW-Z]{2}))|BFPO[ ]?\d{1,4}$`,
@@ -164,9 +167,12 @@ var postCodePatternDict = map[string]string{
 	"YT": `^976\d{2}$`,
 }
 
-var postCodeRegexDict = map[string]*regexp.Regexp{}
+var (
+	postcodeRegexInit sync.Once
+	postCodeRegexDict = map[string]*regexp.Regexp{}
+)
 
-func init() {
+func initPostcodes() {
 	for countryCode, pattern := range postCodePatternDict {
 		postCodeRegexDict[countryCode] = regexp.MustCompile(pattern)
 	}