Panic when refcount overflows

[tmandry]

Instead of silently wrapping, the refcount increment should panic if it overflows.

[hawkw]

For future reference, this is referring to https://github.com/hawkw/sharded-slab/blob/12306dc6c56354e6cb3eea449ab6832d5ed53f8b/src/page/slot.rs#L527