Tag of a single-type SumType wastes space

## maxsamukha (@maxsamukha) reported this on 2024-10-15T12:47:15Z

### Transfered from https://issues.dlang.org/show_bug.cgi?id=24818

### Description

````markdown
import std.sumtype;

static assert(SumType!int.sizeof == 4);
----
Error: static assert:  `8LU == 4LU` is false

No need to store a tag for such SumTypes.
````