Use `unique_ptr` or `shared_ptr` to represent ownership

Reason

They can prevent resource leaks.

Example

Consider:

void f()
{
    X* p1 { new X };              // bad, p1 will leak
    auto p2 = make_unique<X>();   // good, unique ownership
    auto p3 = make_shared<X>();   // good, shared ownership
}

This will leak the object used to initialize p1 (only).

Enforcement

• (Simple) Warn if the return value of new is assigned to a raw pointer.
• (Simple) Warn if the result of a function returning a raw owning pointer is assigned to a raw pointer.