Added assert() to Signal

[Vetnech]

Added a check to make sure :Once, :ConnectOnce and :Connect receive a function so that users do not get confused by the naturally occurring error when they forget to pass a function (like me) or pass something else.

[Sleitnick]

Is there a reason the type-checker didn't catch this issue for you?

[Vetnech]

Is there a reason the type-checker didn't catch this issue for you?

I only received an internal error when the signal was later fired (when the module was looping through the connections and calling each connected function?) which could be confusing as it doesn't traceback to the wrong connection. It personally took a while because the connection without any functions passed was made at a section I had half finished before moving onto a different script.