Simplify on_panic callback handling
Simplify on_panic callback handling

The registration of `panicking::on_panic` as a general-purpose
callback is overcomplicated and can fail.

Instead, invoking it explicitly removes the need for locking and paves
the way for further improvements.

aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust