add node creation framework

Now we've got an easy function to create a node, and templates that allow you to easily choose from some sane defaults. It's also pretty easy to start from a template by making a copy, and then make just a few tweaks to get what you need without having to fill out the whole struct yourself.

The goal here was to make it easy to create a Sia instance.

Next up will be the server package, and then the client package.