make parentindices consistent with Base

[aplavin]

In Julia, the parentindices function returns the indices of a view in its parent array: parentindices(view(A, I)) == (I,). However, for StructArray views this function always returned consecutive indices ignoring the view:

julia> A = [1+2im, 3+4im]
2-element Vector{Complex{Int64}}:
 1 + 2im
 3 + 4im

julia> SA = StructArray(A)
2-element StructArray(::Vector{Int64}, ::Vector{Int64}) with eltype Complex{Int64}:
 1 + 2im
 3 + 4im

julia> parentindices(view(A, [2]))
([2],)

julia> parentindices(view(SA, [2]))
(Base.OneTo(1),)

This PR fixes the above, so that parentindices(view(SA, [2])) == parentindices(view(A, [2])) == ([2],).

[aplavin]

If the implementation is generally fine, I'll fix the allequal error on 1.6.

[piever]

Seems correct to me! Instead of allequal, probably the easiest is to copy the technique used to check equal axes in the constructor

StructArrays.jl/src/structarray.jl

Line 20 in efda46e

map(tail(c)) do ci

(it might even be worth it to factor it out).

[aplavin]

fixed, and added more tests
should be ready to merge

[aplavin]

@piever this PR and several others were merged, but don't seem available in the registered version. Could you please tag a new version?