hvncat: Better handling of 0- and 1-length dims/shape args

[BioTurboNick]

Breaking down #41143 into smaller pieces.

This PR implements handling of 0- and 1-length arguments to dimsshape.

0: enforces single value, returns 0-d array if a scalar or if an array, a copy of the array.

1: dispatches to typed_hcat or typed_vcat depending on row-first and enforces element count.

[simeonschaub]

Thank you very much! Already looks very good, just a couple of small things.

[BioTurboNick]

After thinking about it, I became convinced by @matthias314 's view that hvncat(0) should error; that is, a request to make a 0-dimensional array should require exactly one argument.

Since it's related to that change, I also made it so that hvncat(N) returns an N-dimensional array of 0 size along all those dimensions. It would be the equivalent of [;;] == 0x0 Matrix{Any}, if that were valid syntax. Previously, it would just return an empty vector, which is arguably less consistent.

[JeffBezanson]

Do we really need to allow this? Passing () for 0-d makes sense, but this form doesn't to me, since there is no dimension 0.

[BioTurboNick]

hvncat(::Int, args...) is just a specialization for hvncat(::Tuple{Vararg{Int}}, true, args...), so it continues this pattern:

hvncat(3, ...) == hvncat((1, 1, n), ...).
hvncat(2, ...) == hvncat((1, n), ...)
hvncat(1, ...) == hvncat((n,), ...)
hvncat(0, ...) == hvncat((), ....)

That does mean, though, I could actually consolidate the _typed_hvncat methods for the 0-d cases to all refer to _typed_hvncat(::Type{T}, ::Val{0}, args...), or vice versa.

[JeffBezanson]

Ok, I guess the way to understand it is that it refers to the number of dimensions of the block array?

[BioTurboNick]

Yeah, and it's the special case where there is only one dimension involved: [a ;;; b ;;; c] => hvncat(3, a, b, c) or [a ;;;] => hvncat(3, a), which just bumps up the dimensions if ndims(a) < 3.

[BioTurboNick]

Ah I identified an edge case with a stack overflow.

hvncat(1, [1 2; 3 4])

Occurred because the Int form is checking whether there are higher dimensions in the inputs, which means it has to rely on the more complicated logic of the dims form, but the dims form just sees one dimension and passes it back. Just fixed and added test