struct is_copy_allowed

Generates a type whose ::value is true if |T| is allowed to be copied to/from usermode.

The purpose of this type trait is to ensure a stable ABI and prevent bugs by restricting the

types that may be copied to/from usermode. These types must:

* Be trivial and can be trivially copied.

* Have a standard-layout, which ensures their layout won't change from compiler to compiler.

* Have unique object representations, which ensures they do not contain implicit

padding. Copying types with implicit padding can lead information disclosure bugs because the

padding may or may not contain uninitialized data.