The VDOM release

This release finally switches to using a "virtual DOM" (VDOM) rather than of-screen DOM nodes. The VDOM consists of JavaScript arrays and associative arrays and means a massive speedup. Diffing a 175 kb file took 27 seconds on a modern laptop with version 1.2.0. With version 1.9.0 it takes 1.5-2.2 seconds to diff the same file.

The "random unlimited" tests continues to find bugs after searching for bugs for about 15-30 minutes, but some of the bugs that have been fixed in version 1.9.0 were already present in the 1.2.0 version.

Thanks to @unbug to having contributed with a realworld site integration test.