Use packedstruct instead of unsafe transmuation

[dberlin]

Hey,
Nice crate - i hacked it up a bit to try to make it safer.

Most (but not all) of the unsafe usage is around trying to convert various structures into various byte representations and back.
Or make bitwise copies of non-Copy structures, etc.

For those cases, the packed_struct crate can be used to do it without pain or unsafe code.