Folding string concatenations

[emosaker]

Are there any plans to support string concatenations in the constant folder? I have hacked together a working implementation, though it's "unsafe" as it creates StringRefs out of thin air. I'm not sure about the performance impact as I haven't written enough tests to do a proper benchmark, but I think it's worth looking into.

[vegorov-rbx]

Yes, we are already working on string concat optimizations.
Coming soon, so no need for a PR.