Don't call vec in stack_rest?

[mcabbott]

From https://discourse.julialang.org/t/efficient-way-of-turning-iterator-into-a-matrix/38309:

julia> nums = (i^2 for i in Iterators.filter(_->rand(Bool), 1:100));

julia> Base.haslength(nums)
false

julia> stack(nums)'
1×44 LinearAlgebra.Adjoint{Int64,Array{Int64,1}}:
 1  4  9  49  81  144  196  324  361  400  484  529  …  6241  6561  7396  7744  7921  8649  10000

julia> vecs = ([i,i^2] for i in Iterators.filter(_->rand(Bool), 1:100));

julia> stack(vecs)
2×54 Array{Int64,2}:
 2   7   9   10   11   13   14   16   17   18   20  …    90    92    94    95    96    98    100
 4  49  81  100  121  169  196  256  289  324  400     8100  8464  8836  9025  9216  9604  10000

julia> tups = ((i,i^2) for i in Iterators.filter(_->rand(Bool), 1:100));

julia> stack(tups)
ERROR: MethodError: no method matching vec(::Tuple{Int64,Int64})

julia> stack((i,i^2) for i in  1:100) # with a length, no problem
2×100 Array{Int64,2}:
 1  2  3   4   5   6   7   8   9   10   11   12  …    94    95    96    97    98    99    100
 1  4  9  16  25  36  49  64  81  100  121  144     8836  9025  9216  9409  9604  9801  10000