feature request: Integrate FlexStr

[msparkles]

We have a HashMap<SharedStr, (some data enum)> in our thing that's passed into Rune functions.

We used to use simply String since it worked, but we've found there to be performance bottleneck due to copying strings (not inside of Rune yet, but outside of the environment, we need to pass strings around)

Would it be possible that FlexStr gets integrated? Alternatively, Arc could work too.