mongodb rust driver version: 2.0.0.alpha.1

std::mem::size_of::<mongodb::error::Error>() = 320
std::mem::size_of::<mongodb::error::ErrorKind>() = 296

It's not recommend to pass large object between function,

I think Error or ErrorKind should allocate in heap memory and return a reference on function return value type

`pubkind: ErrorKind` on mongodb::error::Error should change to `pubkind: Box<ErrorKind>`

It's not effective to transfer 320 bytes mongodb::error::Error on stack memory