Resolve clippy warnings in normal build

[zrneely]

These changes fix a few cases where byteorder invokes undefined behavior (caught by clippy) by dereferencing a possibly unaligned pointer (see here).

While I was doing that, I additionally cleaned up the other clippy warnings around the use of the try! macro and using ptr::offset instead of ptr::add.

IMO the only important changes here are the fixes to the UB, so I'm happy to revert the other changes and open a different PR with them (or just forget about them), if you'd prefer.

Edit: I should have looked at the Travis config - looks like 1.12 is the minimum Rust version, so I've reverted my proposed changes around use of try!.

[BurntSushi]

Wow great find! I wonder though if we can perhaps do this without bumping the MSRV?

If we do have to bump the MSRV, then I'm okay with that, since Rust 1.12 is quite old by this point.

[BurntSushi]

Thanks! I merged this manually because the commits were not organized. In the process, I forgot to carry over your author info. Sorry. :-(