Add NCycle iterator

[lstagner]

I think it is useful to be able to set how many times an iterator should be cycled over.

julia> for i=cycle(1:2,2)
           println(i)
       end
1
2
1
2

The documentation should be changed but I am unsure of where to change it.

[tkelman]

The docstring for this is currently at

julia/base/docs/helpdb/Base.jl

Lines 5565 to 5570 in 3f6b2b2

	"""
	cycle(iter)
	An iterator that cycles through `iter` forever.
	"""
	cycle

, but this would be a good opportunity to move it inline next to the source code. If you change the signature in the docstring, you should make the corresponding change to the signature in doc/stdlib/collections.rst then run make docs so the content of the docstring gets translated over.

[lstagner]

I have added tests and documentation. I have also moved the cycle docs inline as you suggested.

[JeffBezanson]

This doesn't work as well as it could since it requires xs to support length.

[lstagner]

Do you have a suggestion on how to improve it? The only way I know of is to create a new iterator type.

[kshyatt]

Well, well, this is your first Julia PR, isn't it @lstagner? Welcome to Julia!

If you could run the doctests as well make -C doc doctest to make sure they all still pass, that would be awesome.

[lstagner]

I have done a rebase against master and all the unrelated doc test failures now pass.

This is ready to merge whenever.

[lstagner]

This PR wasn't getting much traction so I decided to create a custom iterator type instead of using take.

julia> for i=ncycle(1:2,2)
           println(i)
       end
1
2
1
2

[lstagner]

I have rebased and this is ready for review/merging.

I addressed @JeffBezanson comment by making a new iterator type NCycle instead of using take.

[iamed2]

As a new iterator this might fit better in Iterators.jl.

[JeffBezanson]

Are there use cases for this that justify putting it in Base? The existing cycle iterator itself doesn't seem to be used in Base, and with #5155 these things are drawing closer scrutiny.

[lstagner]

If you think Iterators.jl is a better fit for this I will open up a PR there.

[lstagner]

JuliaCollections/Iterators.jl#88