Add llvm::cas::ObjectStore abstraction and InMemoryCAS as a in-memory
CAS object store implementation.
The ObjectStore models its objects as:
* Content: An array of bytes for the data to be stored.
* Refs: An array of references to other objects in the ObjectStore.
And each CAS Object can be idenfied with an unqine ID/Hash.
ObjectStore supports following general action:
* Expected<ID> store(Content, ArrayRef<Ref>)
* Expected<Ref> get(ID)
It also introduces following types to interact with a CAS ObjectStore:
* CASID: Hash representation for an CAS Objects with its context to help
print/compare CASIDs.
* ObjectRef: A light-weight ref for an object in the ObjectStore. It is
implementation defined so it can be optimized for
read/store/references depending on the implementation.
* ObjectProxy: A proxy for the users of CAS to interact with the data
inside CAS Object. It bundles a ObjectHandle and an ObjectStore
instance.
