Features: more arity, string ops

Documentation isn't there yet.  Tests are.

Features added:

 - more arity

 - startswith, endswith, split, slice, implode, explode

 - string slicing

 - string subtraction, multiplication, division, and modulo

   (remove suffix, concat N times, split on separator, and remove prefix)

I think the string subtraction and modulo might be strange, so if you'd like to I can rebase and remove those two.

Note: string slicing currently isn't efficient: it always allocates a new string -- there's no allocation sharing here yet.

Note: string slicing counts codepoints.