Relax dynamic source requirement from Fn to FnMut

[CordlessCoder]

No description provided.

[CordlessCoder]

The second commit should improve performance quite a bit for multithreaded access to Static values, but still requires locking for dynamic closures.
If you think it's worth it, you may restrict Dynamic closures back to Fn instead of FnMut, which will no longer require a Mutex.

[callumio]

Other than that change, looks good - didn't know ArcSwap existed. If you don't want to implement the other methods, I'm happy to do it when I get the chance.

[CordlessCoder]

Took a while to get to it, but should be good now.
I might later try to relax the Clone bound to only apply if you use .set_static, but that might require some unsafe code.

[CordlessCoder]

Done, and I didn't need unsafe :)
Should be ready to merge.

[callumio]

Looks great, thanks!