Fix vector indexing of offset ranges with other ranges

[jishnub]

From the test:

On nightly:

julia> struct ZeroBasedUnitRange{T,A<:AbstractUnitRange{T}} <: AbstractUnitRange{T}
               a :: A
               function ZeroBasedUnitRange(a::AbstractUnitRange{T}) where {T}
                   @assert !Base.has_offset_axes(a)
                   new{T, typeof(a)}(a)
               end
           end

julia> Base.parent(A::ZeroBasedUnitRange) = A.a

julia> Base.length(A::ZeroBasedUnitRange) = length(parent(A))

julia> Base.first(A::ZeroBasedUnitRange) = first(parent(A))

julia> Base.last(A::ZeroBasedUnitRange) = last(parent(A))

julia> Base.size(A::ZeroBasedUnitRange) = size(parent(A))

julia> Base.axes(A::ZeroBasedUnitRange) = map(x -> Base.IdentityUnitRange(0:x-1), size(parent(A)))

julia> Base.getindex(A::ZeroBasedUnitRange, i::Int) = parent(A)[i + 1]

julia> Base.getindex(A::ZeroBasedUnitRange, i::Integer) = parent(A)[i + 1]

julia> Base.firstindex(A::ZeroBasedUnitRange) = 0

julia> function Base.show(io::IO, A::ZeroBasedUnitRange)
               show(io, parent(A))
               print(io, " with indices $(axes(A,1))")
           end

julia> r = ZeroBasedUnitRange(5:8)
5:8 with indices Base.IdentityUnitRange(0:3)

julia> r[0:2]
4:6

After this PR:

julia> r[0:2]
5:7