Sync feature flag

[bogzbonny]

Right now rhai-fs isn't sync (hence it's incompatible with sync users of rhai). Adding a sync feature which replicates Shared and Locked as in rhai and replacing current usages of Rc and RefCell with Shared and Locked would be useful.

[schungx]

Can we just use rhai::Shared and rhai::Locked instead?

I understand that the two APIs are slightly different but I use a few wrapper functions (such as get_shared_lock or something in Rhai to encapsulate them so they look exactly alike in code.

[schungx]

But I'm not sure if a file handle is Send + Sync... therefore does it even make sense to be sync?

[bogzbonny]

Oh yeah that would be cool to just use the primitives directly within rhai. I can confirm that file handling IS sync as I just compiled and used pub type SharedFile = Arc<RwLock<File>>; functions in an async environment

[schungx]

@bogzbonny I suggest that you open a PR if you can.