use forge to implement MD5

This uses the [Forge project](https://github.com/digitalbazaar/forge) to implement MD5, which fixes the failing MD5 test that Marco recently landed.

The problem is that Forge is large, over 800K when using its bundle, which is what I'm doing here.

It's also modular, and the 9K MD5 module only depends on the 75K "utils" module (which we might be able to strip down). But it uses require.js, so I think we'll need to integrate that to use just that module.