Displayed form for vectors of sparse vectors may be improved

[jishnub]

julia> v = sprand(10, 0.3);

julia> [v, v]
2-element Vector{SparseVector{Float64, Int64}}:
   [5]  =  0.782618
  [6]  =  0.482251
   [5]  =  0.782618
  [6]  =  0.482251

This isn't ideal, as it's hard to tell where one ends and the other begins.

[matbesancon]

the advantage of the printing of sparse matrices is that one can use the result to construct another sparse matrix:

julia> print(M)
sparse(Int64[], Int64[], Float64[], 3, 3)

while there is no such thing for vectors, where only the "extended" printing exists

[jishnub]

It's fine to have show(stdout, M) print the full, round-trippable form, and have a compact display for MIME"text/plain" as well as if the IOContext demands this

[matbesancon]

Something like:

"sparsevec($(v.nzind), $(v.nzval), $(length(v)))"