Turn GC allocated string into a scoped heap allocation

[Per Nordlöw]

On Thursday, 13 December 2018 at 13:46:47 UTC, Steven 
Schveighoffer wrote:
> If you use loweredExpr as a key in a builtin AA, then you need 
> to make it a heap allocation, because the GC cleans up AAs.
>
> -Steve

I only need it for lookup not for storage.