Rhai shared scoped refs

[mrazorvin]

No description provided.

[schungx]

A couple of quick observations:

• Try not to change existing API like register_get or else we'll break code. Better to add to the API surface instead.

• Same for examples and tests... Try to add instead of change. This would be quite messy to undo by the time we're ready to merge.

• Try to have as few changed files as possible as that would help with reasoning. Also it'll help putting this under a feature flag.

[schungx]

I wonder how this is working out...