Repr C for zero_copy

[y2kappa]

Fixes #813

[y2kappa]

@armaniferrante @dovahcrow

Not ready yet, would like to see if there are any failing tests.

[fanatid]

I think this can be considered as a breaking change and require a major version bump.

[dovahcrow]

Btw what is the alignment requirement for eBPF? E.g. if I run the VM on a 32bit machine, will it change the alignment requirement from 64to 32?

[archseer]

The VM registers are always 64-bit wide (even when running inside a 32-bit ARM processor kernel!) and support 32-bit subregister addressing [...]

https://www.collabora.com/news-and-blog/blog/2019/04/15/an-ebpf-overview-part-2-machine-and-bytecode/

So looks like 64 bit alignment.

[archseer]

A tip for figuring alignment out: compile with cargo rustc -- -Zprint-type-sizes and then inspect the types for any generated padding.