String interpolation, after a healthy debate on discord

[Ogi]

On Tuesday, 14 December 2021 at 09:35:27 UTC, WebFreak001 wrote:
> no this proposal does not suggest moving any functionality into 
> the compiler. It suggests to add the istrings (tuples with 
> header) as described in the YAIDIP and have special function 
> calling syntax (`functionName"istring contents"`) that will 
> call any function, that accepts an istring as only argument.

I was referring to YAIDIP. According to it, istring should be 
allowed in `mixin`, `pragma(msg)` and `assert` (missed this one). 
This requires reimplementing `text` in the compiler.