Do not support Deref trait on exported newtypes

[agrover]

Being able to deref a Sectors, Datablocks, or MetaBlocks is a hatch for getting around additional (mostly arithmetic) traits being unimplemented for these newtypes. It may be better to add as_u64() or similar to these, to allow explicit conversion, instead of allowing these to be "unwrapped" with a *

[mulkieran]

We can easily add more arithmetic traits. There is no fundamental reason why Bytes has fewer arithmetic traits than Sectors. Ideally, it should be at the point where the only reason for a deref is to pass it as a raw string to an ioct, or something like that.