Preallocate for common case in base58 decode, expose preallocated case

base58.go

@@ -91,6 +91,12 @@ func EncodeMoneroBase58(data ...[]byte) string {
 }
 
 func DecodeMoneroBase58(data string) (result []byte) {
+	//common case
+	return DecodeMoneroBase58PreAllocated(make([]byte, 0, 69), data)
+}
+
+func DecodeMoneroBase58PreAllocated(buf []byte, data string) (result []byte) {
+	result = buf
 	length := len(data)
 	rounds := length / 11
 	for i := 0; i < rounds; i++ {