Inner product using adjoint/transpose returns array instead of scalar

For ```x'*x```, the result should be a scalar, but it currently computes a vector with length 1. We can either:
1. Make ```Axis``` a proper axes type so it can be returned by the ```axes``` function. This should make adjoints/transposes "just work".
2. Keep the adjoint/transpose on the inner array and add multiplication and division rules for ```CArray```s of ```Adjoint```s.