Make on_crash callback FnOnce

[CrabNejonas]

Is your feature request related to a problem? Please describe.
Currently the on_crash callback has to be Fn, which imposes quite heavy restrictions on the closures user are able to pass.

Describe the solution you'd like
If I'm not mistaken the callback will only ever be called once during the lifetime of the instumented app (bc it's gonna crash afterwards) so it should be possible to change the signature to FnOnce to allow consumers maximum flexibility (especially use move closures)

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[Jake-Shadle]

Seems reasonable.