New api: voracious_sort_by?

[Dan-wanna-M]

It is convenient to provide apis that use a lambda to obtain a key rather than implementing a trait, similar to slice::unstable_sort_by.

[lakwet]

mmmh, this sort is heavily optimized.
by using a lambda we would lose all the optimization by type
I could add that, but, I would need to use a default sort (there are several sorts in this lib, and depending on the type you want to sort, the "best" sort is selected) and therefore, decrease performance