Extending bitfields to enumerations of any enumerator type

[Per Nordlöw]

Given

```d
struct S {
	Field field:1;
}
```


, is there a reason why

```d
enum Field : string { none = [], dot = "." }
```

isn't allowed and errors as

```
(1,34): Error: bitfield type `FieldNamePrefix` is not an integer 
type
```

?

Instead I currently have to encode this as

```d
enum Field : ubyte { none, dot }

string toString(in Field arg) pure nothrow @nogc {
	final switch (arg) with (arg) {
	case none: return [];
	case dot: return ".";
	}
}
```

.