Define equality between metrics

[mjp98]

Currently (Distances v0.10.7, Julia 1.8.5) testing equality between metrics parametrised by vectors returns false when comparing two instances, e.g.

julia> m = PeriodicEuclidean(ones(1))
PeriodicEuclidean{Vector{Float64}}([1.0])

julia> m == deepcopy(m)
false

Would it be possible to extend Base.:== and Base.hash for such cases to return true?

(This came up when trying to address JuliaGaussianProcesses/KernelFunctions.jl#492)