Use multi-line format in @show

[matthieugomez]

@show now display objects using the multi-line format.

This pull request fixes #15820 and this Discourse issue. Replaces pull request #22244

[tkelman]

would be good to use do-blocks for some of these, including the redirection so they get put back when finished

[matthieugomez]

Ok I've pushed a new commit doing that. Was it why the tests did not pass?

[tkelman]

it looks like this changes some output that test/test.jl is comparing against

[matthieugomez]

Tests are added and pass.

[tkelman]

(should be squashed, on merge if not before)

[matthieugomez]

Thanks!

[StefanKarpinski]

I think we should consider reverting this. @show is particularly useful for debugging since you can often cut-and-paste the output to recreate the values but this change makes that far less common.

[iamed2]

I don't like how this turned out, for two reasons:

1. @show x now uses a different method than show(x) to display the result
2. @show x was helpful for doctesting for loops in IterTools.jl, but the doctest output would now be so ugly and long that we'll have to change it

I propose bringing back #22244 (@display macro)

Julia 0.6:

julia> show(x)
[1, 2, 3]
julia> show(STDOUT, "text/plain", x)
3-element Array{Int64,1}:
 1
 2
 3
julia> @show x;
x = [1, 2, 3]

Julia 0.7:

julia> show(x)
[1, 2, 3]
julia> show(STDOUT, "text/plain", x)
3-element Array{Int64,1}:
 1
 2
 3
julia> @show x;
x = 3-element Array{Int64,1}:
 1
 2
 3

[StefanKarpinski]

Triage agrees: let's revert this.