Add UUID helper function

UUID creates string literal expression from uuid object.
uuid can be any uuid type with a String method.

mysql/literal.go

@@ -51,6 +51,10 @@ var Decimal = jet.Decimal
 // String creates new string literal expression
 var String = jet.String
 
+// UUID is a helper function to create string literal expression from uuid object
+// value can be any uuid type with a String method
+var UUID = jet.UUID
+
 // Date creates new date literal
 var Date = func(year int, month time.Month, day int) DateExpression {
 	return CAST(jet.Date(year, month, day)).AS_DATE()