Insert at bugfix

[Noobish1]

Hi there, there seems to be an off by one error when inserting at an index when using an Int index. It subtracts twice from the index which leads to it being incorrect. I've removed the subtraction in the insert function which takes a non-int Index as it seems to be correct to expect that to already be off by 1 as you have to explicitly state either head or tail.

I've added a few new tests to go along with it, especially around inserting in the middle of a collection.

[stephencelis]

Thanks for taking the time to make this PR! @mbrandonw and I were recently discussing whether or not this Int-based convenience API is worth the caveats, and we're trying to think through collections that may have Int indices but not be 0-based. We're going to ruminate on it awhile longer, but feel free to let us know what you think!

[Noobish1]

No problem, I have been using this NonEmptyArray implementation and wanted to see how this one compared and the thing I found most awkward/annoying was the enum Index being used, it feels like its leaking implementation details when the consumer of the API shouldn't have to worry about head and tail really. It gets weird when you want to subscript with a range e.g. array[.head...(.tail(4)].