Induced-Norm Convenience Functions

[bhgomes]

It would be nice to be able to get the induced norm from a particular metric space (provided it has a vector-space structure on it). I'm not sure what the API would look like but perhaps evaluating a distance function on only one array runs the norm instead. Something like

v = rand(100)
@assert euclidean(v) == euclidean(v, zero(v))

Right now, running something like euclidean(v, zero(v)) is slower than LinearAlgebra.norm(v), and it would be good to have the possibility to use the Distances API consistently instead of swapping between norm(v) and evaluate(dist, v, 0) for euclidean and other metrics.