Take a `unique_ptr<widget>` parameter to express that a function assumes ownership of a `widget`

Reason

Using unique_ptr in this way both documents and enforces the function call's ownership transfer.

Example

void sink(unique_ptr<widget>); // takes ownership of the widget

void uses(widget*);            // just uses the widget

Enforcement

• (Simple) Warn if a function takes a Unique_pointer<T> parameter by lvalue reference and does not either assign to it or call reset() on it on at least one code path. Suggest taking a T* or T& instead.