std::scoped_lock was introduced in C++17 and is strictly better than
std::lock_guard as it supports locking any number of mutexes safely.
It's also easier to use than std::lock for locking multiple mutexes at
once.
This optimizes the common case of a single simple callback (e.g. std::function
or lambda) so no additional allocation is required. As a Connection return
value is not possible in this case, provide a separate connect_connection()
function to provide that.
Imported from https://github.com/palacaze/sigslot
Classes were renamed from lowercase_me to UppercaseMe style, primarily
to avoid conflicting with the C standard library "signal" function. They
were also moved to the "wpi::sig" namespace.
