Improve performance by making a none multi threaded version of `IString::itern`

[gkorland]

From running some profiling it seems like most of the performance impact vs serde_json is related to the IString::itern.

And most of the overhead is due to the multi threaded code (i.e. locks)

[Diggsey]

Interesting... It would be possible to make the types in this crate !Send unless a feature flag is enabled.

It might also be worth looking at how to improve the multithreaded performance - eg. dashmap 5.x uses parking_lot instead of its own custom lock implementation, so that might provide some performance benefit.

[gkorland]

I tried to upgrade to dashmap 5 but didn't see any impact.