reading from block devices is done using the block cache (bcache).
This cache stores sectors from a block device in pages of memory
marked as 'cached', which will allow them to be reclaimed when
memory pressure is high (TODO).
while block device drivers implement callbacks allowing reading/writing
at block-granularity, the device subsystem uses the block cache to
implement reading/writing at byte-granularity in a driver-agnostic way.
block drivers can disable the block cache for their devices, but this
will require that any clients communicate with the devices at
block-granularity.
also added an offset parameter to device and object read/write functions/callbacks.
this allows the object header to be placed anywhere within the larger
object structure.
the object system now also ensures that the object is zero-initialised
during allocation.
