This optimizes over both the rust rewrite and the original C++ code. The rust
rewrite saw `std::bitset` replaced with `[bool; 65]` which could result in a
lot of memory copy bandwidth each time we checked for and received no signals.
The original C++ code would iterate over all signal slots to see if any were
set. The code now returns a single u64 and only checks slots that are known to
have signals via an intelligent `Iterator` impl.