deprecate nil and cons

[oxinabox]

Slowly getting rid of julia 0.3isms.

Possibly should also deprecate head for first?

[eulerkochy]

You may deprecate head for first. Here's a perspective. These terminologies head and tail of a list, makes sense intuitively. That being said, it would be mentioned in the docs, so an user learning this, probably shouldn't face any major hiccup in getting used to the modified terminology.

[oxinabox]

We are just going to deprecate this type outright I think