relax result_type dist types

[johnnychen94]

I'm working on an extension package to Distances to handle massive pointwise calculation where the output is of size at O(m^2*n^2), and I find it could be quite useful to reuse this result_type here, so this PR is just relaxing it a bit to accept any kind of function dist.

Not very sure if this is a wanted change. I can make up my own result_type function there but I guess to get this done here it would be helpful also to other potential usages.

For Distances own purpose, now it is possible to

julia> result_type(euclidean, Int, Int)
Float64

[johnnychen94]

Without feedback in two weeks, I guess this is probably not a good idea to Distances.jl, so I'm going to close it.

[dkarrasch]

For Distances own purpose, now it is possible to

julia> result_type(euclidean, Int, Int)
Float64

I think this is somehow consistent with the types being callable:

julia> euclidean(2, 3)
1.0

julia> Euclidean()(2, 3)
1.0

julia> result_type(Euclidean(), Int, Int)
Float64

julia> result_type(euclidean, Int, Int)
ERROR: MethodError: no method matching result_type(::typeof(euclidean), ::Type{Int64}, ::Type{Int64})

So, I'm in favor.