[Cocoa] Add a little template magic to the inline API::Object storage to remove the need for all the reinterpret_casts
https://bugs.webkit.org/show_bug.cgi?id=125024

Reviewed by Dan Bernstein.

Introduce API::ObjectStorage which wraps std::aligned_storage and adds some convenience functions
to reinterpret the data as the corresponding type. Deploy it everywhere we were previously using 
std::aligned_storage.

* Shared/Cocoa/WKNSArray.mm:
* Shared/Cocoa/WKNSDictionary.mm:
* Shared/Cocoa/WKObject.h:
* UIProcess/API/Cocoa/WKBackForwardList.mm:
* UIProcess/API/Cocoa/WKBackForwardListItem.mm:
* UIProcess/API/Cocoa/WKBrowsingContextController.mm:
* UIProcess/API/Cocoa/WKBrowsingContextGroup.mm:
* UIProcess/API/Cocoa/WKNavigationData.mm:
* UIProcess/API/Cocoa/WKProcessGroup.mm:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@159881 268f45cc-cd09-0410-ab3c-d52691b4dbfc