Add iterator chain from Iterators.jl

[ghost]

Adding iterators was previously discussed in #10162. However, 'chain' was not included and it is among the features that I miss the most coming from Python and its itertools. All of the code was lifted over from Iterators.jl, I only made some minor modifications to the documentation. As always, feedback is most welcome.

[hayd]

This should be [1:2:5; 0.2:0.1:1.6] (to avoid deprecation message.)

[ghost]

Excellent point, patching and pushing.

[JeffBezanson]

The reason I didn't add this one is that it's too slow. I only wanted the iterators that are basically trivial wrappers, so that there's no overhead over writing the code some other way.

[ghost]

Maybe there is an issue in how I approach problems, a part of me where what is "Pythonic" needs to be stomped out. What I generally use chain for is when I have several collections, say strings from different files, that I need to apply some operation to, such as collecting statistics, yet I need to store them separately in memory. This usually happens during preprocessing, so optimal performance is not that much of an issue. Maybe what I should do is just create a mutating function instead. My apologies for mentioning this on the issue tracker, this may be taking a bit of a "julia-users" turn.

[stevengj]

If something like this is included, it seems like it should accept an iterator of iterators, not just a Vector, as discussed in JuliaCollections/Iterators.jl#50; maybe it should go into the Iterators package, though.

[Keno]

The Flatten iterator with this functionality is in Base now.