Uses dyn Fn instead of dyn FnMut so the callbacks are reentrant

There is a bug in Firefox that can cause a panic when dyn FnMut closures
are used.  https://bugzilla.mozilla.org/show_bug.cgi?id=758004
For my software, simply changing the dyn FnMut to dyn Fn seems to work.
Once this branch is on GitHub, I'll build a version of my software that
I can deploy to where it can get tested by more people.  If it still
works, I'll write up a better explanation and contact scratchyone.