Introduce ASCollectionLayout and friends (#3130)

* Introduce ASCollectionViewLayout
- `ASCollectionViewLayout` is an async `UICollectionViewLayout` that encapsulates its layout calculation logic into a separate thread-safe object which can be used ahead of time and/or on multiple threads.
- `ASDataController` now can prepare for a new layout resulted from a change set before `ASCollectionView` even knows about it. By the time the change set it ready to be consumed by `ASCollectionView`, its new layout is also ready.
- New `ASCollectionViewLayoutCalculating` protocol that is simple and generic enough that many types of calculators can be built on top. `ASCollectionViewLayoutSpecCalculator` conforms to `ASCollectionViewLayoutCalculating` protocol and can be backed by any layout spec (e.g `ASStackLayoutSpec`, `PIMasonryLayoutSpec`, etc). We can even build a `ASCollectionViewLayoutYogaCalculator` that uses Yoga internally.
- A built-in `ASCollectionViewFlowLayoutCalculator` that is a subclass of `ASCollectionViewLayoutSpecCalculator` and uses a multi-threaded multi-line `ASStackLayoutSpec` internally. The result is a performant and thread-safe flow layout calculator.
- Finally, `ASCollectionViewLayout` can be subclassed to handle a specific type of calculator with optimizations implemented based on the knowledge of such calculator. For example, `ASCollectionViewFlowLayout` can have a highly optimized implementation of `-layoutAttributesForElementsInRect:`.

Protocolize layout calculator providing and consuming

Add flex wrap documentation

Add a `multithreaded` flag to ASStackLayoutSpec that forces it to dispatch even if it's off main
- Update ASCollectionViewFlowLayoutSpecCalculator to use that flag.

Minor change in ASCollectionViewLayout

Implement Mosaic layout calculator

Minor change

Fix project file

Rename and fix project file

Skip fetching constrained size only if a layout calculator is available

Update examples/ASCollectionView

Remove unnecessary change in ASTableView

Address comments

Rename collection view calculator protocols

Minor changes after rebasing with master

Add ASLegacyCollectionLayoutCalculator for backward compatibility

Remove ASCollectionLayoutSpecCalculator

Remove ASLegacyCollectionLayoutCalculator

Introduce ASCollectionLayout
- A wrapper object that contains content size and an element to rect table.
- Collection layout calculators to return this new object instead of an ASLayout.

Before adding a content cache

Finishing hooking up ASCollectionLayoutDataSource to ASCollectionNode

Stash

Finish ASCollectionLayout

Rough impl of ASCollectionFlowLayout

Revert changes in CustomCollectionView example

Move ASRectTable back to Private

* Rename ASCollectionContentAttributes to ASCollectionLayoutState

* Address other comments

* Introduce ASCollectionLayoutDelegate and make ASCollectionLayout private

* Address comments

* API tweaks:
- Replace `-layoutContextWithElementMap:` in ASCollectionLayoutDelegate with `-additionalInfoForLayoutWithElements:`. The returned object is then stored in ASCollectionLayoutContext for later lookups.
- ASCollectionLayoutContext has no public initializer.
- ASDataControllerLayoutDelegate no longer requires a context of type ASCollectionLayoutContext but simply an `id`. This helps decouple ASDataController and ASCollectionLayout.
- Rename `elementMap` to `elements`.
- Rename `visibleMap` to `visibleElements`.
- Other minor changes.

* Rename ASCGSizeHash to ASHashFromCGSize

* Make sure to call super in -[ASCollectionLayout prepareLayout]

* Update example/ASCollectionView to use ASCollectionFlowLayoutDelegate

* Remove unnecessary change

Source/Base/ASAssert.h

@@ -59,6 +59,19 @@
 #define ASDisplayNodeCAssertPositiveReal(description, num) ASDisplayNodeCAssert(num >= 0 && num <= CGFLOAT_MAX, @"%@ must be a real positive integer.", description)
 #define ASDisplayNodeCAssertInfOrPositiveReal(description, num) ASDisplayNodeCAssert(isinf(num) || (num >= 0 && num <= CGFLOAT_MAX), @"%@ must be infinite or a real positive integer.", description)
 
+/**
+ * Assert if the current thread owns a mutex.
+ * This assertion is useful when you want to indicate and enforce the locking policy/expectation of methods.
+ * To determine when and which methods acquired a (recursive) mutex (to debug deadlocks, for example),
+ * put breakpoints at some of these assertions. When the breakpoints hit, walk through stack trace frames
+ * and check ownership count of the mutex.
+ */
+#if CHECK_LOCKING_SAFETY
+#define ASDisplayNodeAssertLockUnownedByCurrentThread(lock) ASDisplayNodeAssertFalse(lock.ownedByCurrentThread())
+#else
+#define ASDisplayNodeAssertLockUnownedByCurrentThread(lock)
+#endif
+
 #define ASDisplayNodeErrorDomain @"ASDisplayNodeErrorDomain"
 #define ASDisplayNodeNonFatalErrorCode 1