MHD opposing flow of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"><mml:mi>Cu</mml:mi><mml:mo>−</mml:mo><mml:msub><mml:mi>TiO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math> hybrid nanofluid under an exponentially stretching/shrinking surface embedded in porous media with heat source and slip impacts