DIP: add Bit Fields

[MrSmith]

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

My list of wishes:
1. bitfields imported from C should use `extern(C)`, this way 
they can use whatever layout C compiler uses.
2. D bitfields should be `extern(D)`, and work like `bitfields` 
mixin.
3. Allow specifying default value.

Here is how C3 language does bitfields: 
https://c3lang.github.io/c3docs/types/#bitstructs
It has guaranteed layout, allows specifying endinanness, optional 
overlaping of fields. It allows static arrays to be used as a 
backing type. Its bitfields are done trough a special kind of 
struct - `bitstruct`.