[Sync] Move the last things out of core/.

components/sync/core_impl/README

-This file defines the "sync API", an interface to the syncer
-backend that exposes (1) the core functionality of maintaining a consistent
-local snapshot of a hierarchical object set; (2) a means to transactionally
-access and modify those objects; (3) a means to control client/server
-synchronization tasks, namely: pushing local object modifications to a
-server, pulling nonlocal object modifications from a server to this client,
-and resolving conflicts that may arise between the two; and (4) an
-abstraction of some external functionality that is to be provided by the
-host environment.
-
-This interface is used as the entry point into the syncer backend
-when the backend is compiled as a library and embedded in another
-application.  A goal for this interface layer is to depend on very few
-external types, so that an application can use the sync backend
-without introducing a dependency on specific types.  A non-goal is to
-have binary compatibility across versions or compilers; this allows the
-interface to use C++ classes.  An application wishing to use the sync API
-should ideally compile the syncer backend and this API as part of the
-application's own build, to avoid e.g. mismatches in calling convention,
-structure padding, or name mangling that could arise if there were a
-compiler mismatch.
-
-The schema of the objects in the sync domain is based on the model, which
-is essentially a hierarchy of items and folders similar to a filesystem,
-but with a few important differences.  The sync API contains fields
-such as URL to easily allow the embedding application to store web
-browser bookmarks.  Also, the sync API allows duplicate titles in a parent.
-Consequently, it does not support looking up an object by title
-and parent, since such a lookup is not uniquely determined.  Lastly,
-unlike a filesystem model, objects in the Sync API model have a strict
-ordering within a parent; the position is manipulable by callers, and
-children of a node can be enumerated in the order of their position.