String Interpolation

[Commander Zot]

can we please just have ```i"whatever ${var}"``` to return a 
fully interpolated string, not a tuple, not a template or 
anything.
because the common use case is ```auto s = i"whatever ${var}";```,
so maybe just lower it into ```format("whatever %s", var)```.

Then also introduce ```t"whatever ${var}"``` for a tuple/template 
returned object as a completely seperate proposal and you discuss 
whatever proposal is better for it YADIP or 1037 or whatever.

this way it's also a lot simpler to understand what is happening 
for the users.