* Move seal barrier type field from Access to autoSeal struct.
Remove method Access.SetType(), which was only being used by a single test, and
which can use the name option of NewTestSeal() to specify the type.
* Change method signatures of Access to match those of Wrapper.
* Turn seal.Access struct into an interface.
* Tweak Access implementation.
Change `access` struct to have a field of type wrapping.Wrapper, rather than
extending it.
* Add method Seal.GetShamirWrapper().
Add method Seal.GetShamirWrapper() for use by code that need to perform
Shamir-specific operations.
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