The sync feature flag is not compatible with no_std

[roeeshoshani]

when i enable both the sync and the no_std feature flags, rhai won't compile due to using std::sync::RwLock, which is not provided by the no-std-compat crate due to not using the compat_sync feature flag of that crate.

this can simply be solved by enabling the compat_sync feature flag of the no-std-compat crate.

[schungx]

Ah, thanks for this. Will you make a PR? Or I can do it.