union #shared_nil

This adds a feature to `union` which requires all the variants to have a `nil` value and on assign to the union, checks whether that value is `nil` or not. If the value is `nil`, the union will be `nil` (thus sharing the `nil` value)

core/runtime/core.odin

@@ -136,6 +136,7 @@ Type_Info_Union :: struct {
 	custom_align: bool,
 	no_nil:       bool,
 	maybe:        bool,
+	shared_nil:   bool,
 }
 Type_Info_Enum :: struct {
 	base:      ^Type_Info,