future proofing: use ManuallyDrop rather than mem::forget for callback management

[pnkfelix]

Feedback from zulip is questioning how I'm handling the forgetting of the callback type, namely Elichai Turkel notes:

Doesn't this moves the callback type from the stack and thus invalidating the pointer? IMHO it's easiest to use a ManuallyDrop

cee-scape/src/lib.rs

Line 314 in 1ce76e4

std::mem::forget(callback);

[pnkfelix]

See also discussion on rust-lang/unsafe-code-guidelines#188