Function abs

tests/test_operator_overloading.cpp:133–133

Not a good abs function, but easy to test.

// Not a good abs function, but easy to test.
std::string abs(const Vector2 &) { return "abs(Vector2)"; }

// clang 7.0.0 and Apple LLVM 10.0.1 introduce `-Wself-assign-overloaded` to
// `-Wall`, which is used here for overloading (e.g. `py::self += py::self `).