Use/require byte equality in VarZeroVec

[Manishearth]

Part of #1082

Fixes #1083

[sffc]

Please add the default implementation of the two trait functions where that is possible, and remove the overrides from the impls where possible.

[sffc]

Please add a default impl for from_byte_slice_unchecked as well. For ULE, you can use mem::size_of to figure out how long to make the resulting slice. For VarULE, I think you can just cast it, right?

[Manishearth]

You can't cast it for VarULE because there's no way to generically do the inverse operation of size_of_val() ("I have X bytes, tell me what the pointer metadata should be"). For example, consider what would happen with stuff like struct ComplexULE(u8, char::ULE, [u32::ULE]), calculating the pointer metadata for that is very specific to that type.

Now fortunately the only kind of DST we can theoretically support are slice DSTs and compound slice DSTs, but just to give an example of why the inverse of size_of_val() is untractable: how woudl you solve that for trait objects? At least for compound slice-based DSTs rustc could theoretically add APIs to std::ptr::metadata that let you do this.