Remove unused pure curried functions where possible

With Rollup v0.41.4, using the following input:

function foo(a) {
  return function(b) {
    return a+b;
  }
}

function bar(a, b) {
  return a+b;
}

foo(1)(2);
bar(1,2);

Note that foo and bar are the same function except that foo is curried.

When bundling this as ES2015, I hoped to see an empty module, but got this:

function foo(a) {
  return function(b) {
    return a+b;
  }
}

foo(1)(2);

If this is something you'd like Rollup to be able to do, I'd be interested in working on this.

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