Merge reactive scope refactor

docs/tut/crash-course/7-cleanup.md

@@ -0,0 +1,40 @@
+# Cleanup
+
+Sometimes you may need to do some cleanup when destroying a component or after
+a side-effect from a source update. Vide provides a function `cleanup()` which
+is used to register a cleanup callback for the next time the reactive scope
+it is called in re-runs.
+
+```lua
+local vide = require(vide)
+local source = vide.source
+local cleanup = vide.cleanup
+
+local function Timer()
+    local count = source(0)
+
+    local con = game:GetService("RunService").Heartbeat:Connect(function(dt)
+        count(count() + dt)
+    end)
+
+    cleanup(function()
+        con:Disconnect()
+    end)
+
+    return create "TextButton" {
+        Position = UDim2.fromOffset(300, 300),
+        Size = UDim2.fromOffset(200, 50),
+
+        Text = function()
+            return "seconds: " .. count()
+        end,
+    }
+end
+
+mount(function() return create "ScreenGui" { Timer {} } end, game.StarterGui)
+```
+
+In the above example, this allows us to disconnect the heartbeat connection
+when the timer component is destroyed, whether that is from unmounting the app
+or if it is dynamically created by a control-flow function, which will be
+covered next.