Indexing SOneTo with IdentityUnitRange preserves indices

[jishnub]

On master:

julia> SOneTo(3)[Base.IdentityUnitRange(2:3)]
2:3

After this PR:

julia> SOneTo(3)[Base.IdentityUnitRange(2:3)]
Base.IdentityUnitRange(2:3)

This makes it consistent with julia's vector indexing, eg. for Base.OneTo (JuliaLang/julia#40997)

[jishnub]

After 47ce9a0:

julia> sm = @SMatrix [1 3; 2 4]
2×2 SMatrix{2, 2, Int64, 4} with indices SOneTo(2)×SOneTo(2):
 1  3
 2  4

julia> sm[SOneTo(length(sm))]
4-element SVector{4, Int64} with indices SOneTo(4):
 1
 2
 3
 4

julia> view(sm, map(Base.IdentityUnitRange, axes(sm))...) |> axes
(SOneTo(2), SOneTo(2))

[jishnub]

After c53a671 vector indexing may reshape a StaticArray if necessary

julia> sm = @SMatrix [1 3; 2 4]
2×2 SMatrix{2, 2, Int64, 4} with indices SOneTo(2)×SOneTo(2):
 1  3
 2  4

julia> sm[:, SOneTo(1), 1, SOneTo(1)]
2×1×1 SArray{Tuple{2, 1, 1}, Int64, 3, 2} with indices SOneTo(2)×SOneTo(1)×SOneTo(1):
[:, :, 1] =
 1
 2

One side-effect is that the error message for out-of-bounds indexing becomes a bit confusing as we don't perform the bounds checking at the top level:

julia> sm[:, SOneTo(1), 1, SOneTo(2)]
ERROR: BoundsError: attempt to access 2×2×1×1 SArray{Tuple{2, 2, 1, 1}, Int64, 4, 4} with indices SOneTo(2)×SOneTo(2)×SOneTo(1)×SOneTo(1) at index [1, 1, 1, 2]

However this may be changed.