Fix Clazy warnings (#4988)

* stl.h: Use C++11 range-loops with const reference

This saves copy-ctor and dtor for non-trivial types by value
Found by clazy (range-loop-reference)

* test_smart_ptr.cpp cleanup

Introduce `pointer_set<T>`

https://github.com/boostorg/unordered/issues/139



> Based on the standard, the first move should leave a in a "valid but unspecified state";

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Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>