Potentially the whole `pow` module will move to `primitives` but this
is not possible easily right now. However, we would like to be able to
move the `BlockHash` and `block::Header` types over to `primitives`
and doing so requires the `CompactTarget` to be there.
Move the `CompactTarget` type to `primitives` and re-export it from the
`primitives` crate root.
Note also, we re-export the type publicly from `bitcoin::pow`.
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