Expose occupied() in unordered map

[david-tingdahl-nvidia]

Useful when iterating over the whole hash and copying valid entries.

[stotko]

Thanks for the feature request. The same change has also been proposed in #427 to address #423. As already mentioned there, I believe that publicly exposing the internal occupied() function is not the right design. Instead, device_range() should be used to iterate over the (compressed) set of values which is more efficient, especially at low load factors.