Auto merge of rust-lang#147784 - nxsaken:div_exact_return_option, r=dtolnay

Return `Option` from `exact_div` and inherit overflow checks

According to rust-lang#139911 (comment), `exact_div` should return `Option::None` if `self % rhs != 0`, panic if `rhs == 0`, and handle overflow conditionally (panic in debug, wrap in release).

rust-lang#147771 should rename `exact_div` to `div_exact`.

Author: bors