Collection.js imports Seq.js and Seq.js imports Collection.js

Because circular dependencies aren't supported by esm, the files aren't usable as-is and must be transpiled.

Also, these lines in Seq.js look strange.

if ( Collection ) Seq.__proto__ = Collection;
Seq.prototype = Object.create( Collection && Collection.prototype );
Seq.prototype.constructor = Seq;

When Collection is imported through a circular dependency, its value is falsey undefined which causes a runtime error when it is passed to Object.create

related #1961