Curry with Placeholders

Implement curry(fn), which turns a function into a curried one that accepts its arguments in several calls, with placeholder support.

• The curried function can be called with any number of arguments at a time. As soon as enough arguments have been collected (at least fn.length, and none of the first fn.length is a placeholder), it calls fn with all collected arguments and returns the result. Until then it returns a new function that keeps collecting.
• curry.placeholder is a unique value that marks a skipped argument. A placeholder in a call reserves that position; later arguments fill the earliest reserved positions first, and only then are appended.
• Partial applications are reusable: calling a returned function twice with different arguments must not interfere.
• this of the final call is passed to fn.