do not wrap default-exported functions in object literals

[bakkot]

On master, export default (function f (){}); compiles to

(function(e) {
  'use strict';
  var w = { default: function f() {} }.default;
  var r = { __proto__: null, default: w };
  if (e.Symbol)
    e.Object.defineProperty(r, e.Symbol.toStringTag, { value: 'Module' });
  r = e.Object.freeze(r);
  return r;
})(this);

Now it compiles to

(function(e) {
  'use strict';
  var w = function f() {};
  var r = { __proto__: null, default: w };
  if (e.Symbol)
    e.Object.defineProperty(r, e.Symbol.toStringTag, { value: 'Module' });
  r = e.Object.freeze(r);
  return r;
})(this);

There's also a test that default exports work, because I'm not sure they were sufficiently stressed before.

Contains version bump commit, so please rebase, not squash.

[bakkot]

It's plausible this breaks function name inference in some cases, but it's already pretty broken (see #64), so I don't think we should worry about that.