Using `to_dimension` for dimension specific methods

[Tokazama]

I think it would be useful if we had a formally defined to_dimension method that did something like:

@propagate_inbounds function to_dimension(A::AbstractArray{T,N}, d::Integer) where {T,N}
    @boundscheck if d < 1
        error("BoundsError: attempt to access $(typeof(A)) at dimension $d")
    end
    return Int(d)
end

@propagate_inbounds function to_dimension(A::AbstractArray{T,N}, dims::Tuple{Vararg{<:Integer,L}}) where {T,N,L}
    return ntuple(i -> to_dimension(A, getfield(dims, i)), Val(L))
end

This would provide more useful error messages and provide another useful hook into the array interface. For example, methods that have a dimension argument could easily be made type stable with the following:

@propagate_inbounds function to_dimension(A::AbstractArray, ::Val{D}) where {D}
    return to_dimension(A::AbstractArray, D)
end

# 
function dropdims(A; dims)
    d = to_dimension(A, dims)
    ....
end