[Issue 19896] [internals] Represent string and arrays as sparse literals

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19896

--- Comment #2 from Iain Buclaw <ibuclaw at gdcproject.org> ---
(In reply to Nicholas Wilson from comment #1)
> I think we should be fast and memory efficient for the common case, and most
> string aren't sparse. What use case is that?

I'm willing to admit that StringExp may be the wrong target to try to fix. 
ArrayLiteralExp's however I won't give up on.

Casting a StringExp to any array type shouldn't create a new (potentially
bigger) StringExp.

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