Change function_exists to &self

[milesj]

I wrap the Plugin instance in a RwLock and because function_exists requires &mut self, I have to acquire a write lock everytime to just check if a function exists, which causes lock contention.

This changes it to &self since it doesn't need to mutate.

[bhelx]

Thank you :)

Will have @zshipko peek at it on Monday

[zshipko]

Thanks! Test failures are clippy related, I will fix those in a follow up PR.