Return PCode slice contains the same value for each element due to ZSTs

[chc4]

RustPCodeEmitProxy::dump passes the slice start pointer and length to Rust, which converts it into a slice with vars: &[sleigh_sys::ffi::VarnodeData]. The VarnodeData type is declared as a zero-sized type for cxx FFI, however, and not a struct of the same size as what the C++ side has: this means that when you loop over the slice in Rust, each element is actually the start pointer length times, since it advances by 0 each time. This is visible in black-binary/sleigh#1 for example, where each varnode for the store operation is the address space varnode instead of [address space, pointer, data] like it should be.

I have a fork where I fixed the issue by having the C++ layer return a vars: &[&sleigh_sys::ffi::VarnodeData] instead, since advancing by pointer size is fine and allows it to continue using a ZST FFI type. It's mixed in with other things I added when messing around, however, and needs to be pulled out individually: chc4@498daa1 chc4/sleigh@3011940