DIP: add Bit Fields

[Nick Treleaven]

On Thursday, 7 March 2024 at 04:26:56 UTC, Walter Bright wrote:
> https://github.com/WalterBright/documents/blob/master/bitfields.md

> One cannot take a reference to or the address of a bitfield

Taking a ref is not detected yet:
https://issues.dlang.org/show_bug.cgi?id=24263

I was wondering how to read/write the bits of a bitfield set as 
their underlying type. So I tried putting a bitfield set inside 
an anonymous union:

```d
import std.stdio;

struct S
{
     union
     {
         uint u;
         uint a:1,
              b:1,
              c:1;
     }
}
static assert(S.sizeof == 4);

void main()
{
     S s;
     s.tupleof.writeln(); // 0000
     s.u++;
     s.tupleof.writeln(); // 1111
}
```
I was a bit surprised that setting `u` to 1 sets all 3 bitfields 
to 1 as well - I was assuming the union had 2 uint members where 
the 2nd one didn't have a name. Then I realized I needed to put 
the bitfields inside an anonymous struct, because otherwise each 
one was a union member. Anyway doing that does work.