When packaging, copy the code and inline `__DEV__`

Fixes #812.

Previously, this code

  module.exports = moo();
  function moo() { return __DEV__; }

would be transformed to

  module.exports = moo();
  function moo() { return "production" !== process.env.NODE_ENV; }

Now, it's transformed to:

  if ("production" !== process.env.NODE_ENV) {
    var moo = function() { return true; };
    module.exports = moo();
  } else {
    var moo = function() { return false; };
    module.exports = moo();
  }

which reduces the getter cost to one test at require time instead of inline for every `__DEV__` check, warning, and invariant.

The unminified build is about twice as large now (but it's about the same after gzipping) and the minified build is the same size.