Expose way to bypass trivial destr/move on extern types passed by value

[dtolnay]

// IsRelocatable<T> is used in assertions that a C++ type passed by value
// between Rust and C++ is soundly relocatable by Rust.
//
// There may be legitimate reasons to opt out of the check for support of types
// that the programmer knows are soundly Rust-movable despite not being
// recognized as such by the C++ type system due to a move constructor or
// destructor. To opt out of the relocatability check, do either of the
// following things in any header used by `include!` in the bridge.
//
//      --- if you define the type:
//      struct MyType {
//        ...
//    +   using IsRelocatable = std::true_type;
//      };
//
//      --- otherwise:
//    + template <>
//    + struct rust::IsRelocatable<MyType> : std::true_type {};
template <typename T>
struct IsRelocatable;

Closes #359.