introduce a dedicated AST type

[zerbina]

The goal is to separate the generic node storage and traversal operations from the specific concept of an abstract syntax tree (=AST).

An AST requires some extra bits of information (e.g., metadata), but adding this to PackedTree would make the type it no longer applicable for use cases that want just a sequence of nodes and the associated operations without anything else.

While there currently are no such use cases, I can foresee that changing in the future, so splitting the two things makes sense.

As is, the current code is a dead end. Conceptually, the node tree operations (e.g.: child, next, etc.) do not care about the concrete storage type -- they only require an input type that's a random-access supporting container type whose element is a type with a mutable kind and val property, nothing more. This makes them an ideal candidate for static interfaces (i.e., concept), but said NimSkull feature is not yet stable enough.

I've tried to approximate the ideal solution (static interfaces) by making PackedTree type only represent the node storage and have it be parameterized with the node type, with Ast then embedding a PackedTree instance and providing a wrapper template for every PackedTree routine such that Ast can be used in the same way a PackedTree can be, but this is annoying to maintain and it also significantly hurts code clarity, since various routines for Builder, ChangeSet, etc. have to use unnecessarily broad interfaces.

Using inheritance would work a little better in practice, but is not really correct (an Ast cannot be used everywhere a PackedTree can be, for example, serialization), and it would likely also surface some NimSkull bugs/issues with argument subtyping and iterators.

Progress on this PR needs to wait until NimSkull supports working static interfaces. In the meantime, PackedTree will have to become the Ast type.

[zerbina]

This refactor will be made obsolete with the eventual arrival of #179.