Break apart plugin reconciler (#332)

* Start splitting apart reconciler, with tests

* Reify children in reify

* Baseline hydrate implementation

* Remove debug print

* Scaffold out diff implementation, just supporting name changes

* invariant -> error in decodeValue

* Flesh out diff and add getProperty

* Clear out top-level reconciler interface, start updating code that touches it

* Address review feedback

* Add (experimental) Selene configuration

* Add emptiness checks to PatchSet, remove unimplement invert method

* Improve descendant destruction behavior in InstanceMap

* Track instanceId on all reify errors

* Base implementation of applyPatch, returning partial patches on failure

* Change reify to accept InstanceMap and insert instances into it

* Start testing applyPatch for removals

* Add test for applyPatch adding instances successfully and not

* Add , which is just error with formatting

* Correctly use new diff and applyPatch APIs

* Improve applyPatch logging and fix field name typo

* Better debug output when reify fails

* Print out unapplied patch in debug mode

* Don't write properties if their values are not different.

This was exposed trying to sync the Rojo plugin, which
has a gigantic ModuleScript in it with the reflection
database. This workaround was present in some form in
many versions of Rojo, and I guess we still need it.

This time, I actually documented why it's here so that
I don't forget for the umpteenth time...

* Add placeholder test that needs to happen still

* Introduce easier plugin testing, write applyPatch properties test

* Delete legacy get/setCanonicalProperty files

* Fix trying to remove numbers instead of instances

* Change applyPatch to return partial patches instead of binary success

* Work towards being able to decode and apply refs

* Add helpers for PatchSet assertions

* Apply refs in reify, test all cases

* Improve diagnostics when patches fail to apply

* Stop logging when destroying untracked instances, it's ok

* Remove read before setting property in applyPatch

* Fix diff thinking all properties are changed

plugin/src/InstanceMap.lua

@@ -33,6 +33,16 @@ function InstanceMap.new(onInstanceChanged)
 	return setmetatable(self, InstanceMap)
 end
 
+function InstanceMap:size()
+	local size = 0
+
+	for _ in pairs(self.fromIds) do
+		size = size + 1
+	end
+
+	return size
+end
+
 --[[
 	Disconnect all connections and release all instance references.
 ]]
@@ -81,8 +91,6 @@ function InstanceMap:removeId(id)
 		self:__disconnectSignals(instance)
 		self.fromIds[id] = nil
 		self.fromInstances[instance] = nil
-	else
-		Log.warn("Attempted to remove nonexistant ID {}", id)
 	end
 end
 
@@ -93,8 +101,6 @@ function InstanceMap:removeInstance(instance)
 	if id ~= nil then
 		self.fromInstances[instance] = nil
 		self.fromIds[id] = nil
-	else
-		Log.warn("Attempted to remove nonexistant instance {}", instance)
 	end
 end
 
@@ -102,10 +108,14 @@ function InstanceMap:destroyInstance(instance)
 	local id = self.fromInstances[instance]
 
 	if id ~= nil then
-		self:destroyId(id)
-	else
-		Log.warn("Attempted to destroy untracked instance {}", instance)
+		self:removeId(id)
 	end
+
+	for _, descendantInstance in ipairs(instance:GetDescendants()) do
+		self:removeInstance(descendantInstance)
+	end
+
+	instance:Destroy()
 end
 
 function InstanceMap:destroyId(id)
@@ -113,21 +123,11 @@ function InstanceMap:destroyId(id)
 	self:removeId(id)
 
 	if instance ~= nil then
-		local descendantsToDestroy = {}
-
-		for otherInstance in pairs(self.fromInstances) do
-			if otherInstance:IsDescendantOf(instance) then
-				table.insert(descendantsToDestroy, otherInstance)
-			end
-		end
-
-		for _, otherInstance in ipairs(descendantsToDestroy) do
-			self:removeInstance(otherInstance)
+		for _, descendantInstance in ipairs(instance:GetDescendants()) do
+			self:removeInstance(descendantInstance)
 		end
 
 		instance:Destroy()
-	else
-		Log.warn("Attempted to destroy nonexistant ID {}", id)
 	end
 end