Make length.(keys) work

[cossio]

Try this:

d = Dict([1,2] => 1.1, [3,2] => 0.1)
length.(keys(d))

It doesn't work on 0.5 (I've been told it works on master, https://discourse.julialang.org/t/length-keys-d-does-not-work/913). Can we pull this into 0.5.1?

[nalimilan]

Looks like I was mistaken. Here's what this returns on master:

julia> length.(keys(d))
2

So actually it just falls back to length(keys(d)). That's a more general problem with iterators, as length.(1:2 for i in 1:3) returns 3 rather than [2, 2, 2] too. I guess that's a matter of defining broadcast_indices for these objects.

Ironically, it's easier to fix on 0.5 than on master, since defining Base.size(x::Base.KeyIterator) = (length(x),) is enough for the former, but not for the latter.

[mbauman]

Mostly a duplicate of #20678/#10092.