std.format and uninitialized elements in CTFE

[Johan Engelen]

Uninitialized member variables, or partially initialized structs, 
are needed for e.g. small-string optimized string types.
There are a number of bug reports about it, see: 
https://issues.dlang.org/show_bug.cgi?id=11331

I remember that at the end of my DConf talk in 2017, we got 
Walter's approval to treat `=void` initialized struct members as 
if they are initialized / skip all initialization of it.
- Discussion of =void: https://youtu.be/YL6Tp8Zb5aI?t=2959
- Confirmation of that we should do something about it: 
https://youtu.be/YL6Tp8Zb5aI?t=3084
- Approval :-)  https://youtu.be/YL6Tp8Zb5aI?t=3266

This is a fun project to work on for LDC: 
https://github.com/ldc-developers/ldc/issues/3249

-Johan