Remove unsafe from the request parsing code

[wmanley]

Instead we use zerocopy::FromBytes.

Previously the code did not ensure alignment, which could lead to undefined behaviour. Now we do check alignment I've had to fix it in a few places using AlignedData<[u8; sz]> as a buffer in tests. Unfortunately we can't use Box<AlignedData<u8; BUFFER_SIZE>> as our main buffer because it's too large to create on the stack and Rust currently offers no way to allocate it directly on the heap. Instead we have to fiddle with offsets for the Vec.

[wmanley]

@cberner: I consider this mergable. Please let me know if you want any changes. There are no API changes here.

[cberner]

Great! I'll take a look soon

[wmanley]

Thanks for the review. I've fixed the issues and force-pushed.

[cberner]

Merged. Thanks!