Add createShared() and createUnique() pattern

[jslee02]

This PR adds a set of macros that defines two static functions ( createShared and createUnique) for creating shared and unique instances of the object. Developers must carefully choose which macro should be used depending on two factors: (1) whether the object requires aligned memory allocation and (2) whether the object's constructor is public or protected. Whereas, the users are encouraged to use createShared and createUnique without worrying about those factors if the static creators are defined.

Related issue: #813

Open Decisions

• Name for shared object creator: create() vs createShared(): We use create and createShared for raw pointer and std::shared_ptr, respectively

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This change is 

[psigen]

I like this change! 👍

We should probably consider a followup PR that documents the new macros and static create* functions for future developers.