Check which adapters can tweak projection instead of comparison

[Morwenn]

Sorters that can take a custom comparison function can implicitly take a projection function too, so sorter adapters can be more widely used if they tweak the behaviour of the projection function instead of that of the comparison function.

It looks like at least indirect_adapter can benefit from such an improvement. Implement that and check whether other sorter adapters can also take advantage of that.

[Morwenn]

Apparently indirect_adapter was the only one that could actually be improved: scwhartzian_adapter already uses the projection while it seems hard to remove the comparison in stable_adapter.