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/Reconciler/init.lua

@@ -0,0 +1,34 @@
+--[[
+	This module defines the meat of the Rojo plugin and how it manages tracking
+	and mutating the Roblox DOM.
+]]
+
+local applyPatch = require(script.applyPatch)
+local hydrate = require(script.hydrate)
+local diff = require(script.diff)
+
+local Reconciler = {}
+Reconciler.__index = Reconciler
+
+function Reconciler.new(instanceMap)
+	local self = {
+		-- Tracks all of the instances known by the reconciler by ID.
+		__instanceMap = instanceMap,
+	}
+
+	return setmetatable(self, Reconciler)
+end
+
+function Reconciler:applyPatch(patch)
+	return applyPatch(self.__instanceMap, patch)
+end
+
+function Reconciler:hydrate(virtualInstances, rootId, rootInstance)
+	return hydrate(self.__instanceMap, virtualInstances, rootId, rootInstance)
+end
+
+function Reconciler:diff(virtualInstances, rootId)
+	return diff(self.__instanceMap, virtualInstances, rootId)
+end
+
+return Reconciler