Use `make_unique()` to make `unique_ptr`s

Reason

make_unique gives a more concise statement of the construction. It also ensures exception safety in complex expressions (in pre-C++17 code).

Example

unique_ptr<Foo> p {new Foo{7}};    // OK: but repetitive

auto q = make_unique<Foo>(7);      // Better: no repetition of Foo

Enforcement

(Simple) Warn if a unique_ptr is constructed from the result of new rather than make_unique.