Make BlockIo hidden in GblOps

libgbl should not really care about the actual type of PartitionBlockIo
since there is no API that needs to return values containing this
type back to the caller. The CL makes the type hidden and removes the
associated type PartitionBlockIo and the `Self:'a` constraint.

Change-Id: Ic74f28427d7ca9f2dc2c966a86be4b8f4e629d9b