fix undef showing

[Moelf]

main issue was that error happens at assign:

Pluto.jl/src/runner/PlutoRunner.jl

Line 372 in b53d282

i, el = pair

so instead of broadcast over zip of (index, elements), a specialized function for array is implemented.

Also, I think a lot can be re-used from Base, for example:

julia> Base.print_matrix(stdout, vvv)
 #undef
 #undef
 #undef

Maybe for the next re-factor 👷

Also I switched to @view when possible, since users can have huge array (maybe each element is big or something), so slicing with copy was not as good of an idea (I think).

After this patch:

[Moelf]

fix #352

[fonsp]

Nice! Could you add a testset?

https://github.com/fonsp/Pluto.jl/blob/master/test/RichOutput.jl

[Moelf]

@fonsp done

[fonsp]

Can you remove abbreviations? idx, ary, els

[Moelf]

Can you remove abbreviations? idx, ary, els

yes, where and why? I mean we used i, el = pair before without a problem right?

[fonsp]

Awesome, thanks!