Add promote_rule for OneTo and AbstractUnitRange

[jishnub]

We may assume that a OneTo behaves like a UnitRange on promotion. With this, custom AbstractUnitRange types don't need to define promotion rules with OneTo, and the following works out of the box:

julia> struct MyUnitRange{T} <: AbstractUnitRange{T}
               range::UnitRange{T}
       end

julia> Base.first(r::MyUnitRange) = first(r.range)

julia> Base.last(r::MyUnitRange) = last(r.range)

julia> Base.size(r::MyUnitRange) = size(r.range)

julia> Base.length(r::MyUnitRange) = length(r.range)

julia> Base.getindex(r::MyUnitRange, i::Int) = getindex(r.range, i)

julia> promote(MyUnitRange(3:4), Base.OneTo(4))
(3:4, 1:4)

There is some potential for ambiguity if a package defines promote_rule(::AbstractUnitRange, ::CustomUnitRange), in which case a working promote_rule(::OneTo, ::CustomUnitRange) becomes ambiguous. However, a similar problem exists for UnitRange as well, and I think the benefits outweigh the risks.