Head and tail about nspl

Apparently init is also missing, which returns all elements but the last, so it's the reverse of last and does what tail does, but for the last instead of the first element.

See also here or this very enlightening picture (by Miran Lipovača):

[ihor]

Hey! Thanks for your suggestions. The functions are missing because I never needed them. I started this library when I was also working with Python and tried to copy some of the things I liked to PHP (and fix the weird API of the standard library).

I'll review @tominventisbe 's PRs this weekend.

[Lambik]

@ihor I suppose "this weekend" has already passed, and there are 3 PRs open that seem nice :-) Not that I need them right this instant or something, but for completion of the library's sake, could you review them? I like the zip-one for instance.

[jtojnar]

@ihor I can help maintain the library if you do not have the time.