fn* and multiple arity functions

[alandipert]

• fn should be renamed fn*
• fn* should support multiple, different arity versions of the same function, as in Clojure
• Possibly: provide an fn that supports sequence destructuring in its argument list
  • Implies a destructuring function we could use elsewhere, like on the left-hand side of let bindings.

It's good because it's a nice way to provide default arguments, particularly when seeding lazy sequences or recursion.

Open questions

• Perf?
  • An additional special to speed up arity selection of inlines?
    • That way we could call through to the right implementation without consulting an arities -> implementations table.