Support node API

[Techcable]

This is a higher-level API that basically treats mpack data as a bunch of untyped nodes.

We might want to consider writing our own API instead of bindings to the C API. That could more in line with idiomatic Zig (tagged unions....)

This is lower priority for me personally because my data tends to be in a fixed schema, or quickly converted into some other form.

As mentioned in the README, this is less important than it is in C because Zig has better error handling and type-reflection.

[Techcable]

I don't want to try and implement struct deserialization with type reflection (at least not anytime soon).

This makes this node API more important.