hump.signal - crashes when sending "clear", "remove" or any other signals that share a name with a Registry function name.

signal.emit("clear")

Error: library/hump/signal.lua:42: bad argument #1 to 'pairs' (table expected, got function)

```LUA
function Registry:emit(s, ...)
	for f in pairs(self[s]) do
		f(...)
	end
end
```
Registry:emit does not check if ```self[s]``` is a function before trying to read it like a table.