This is the same infra as ffigen uses: #1638.

• Copied the AstNode classes etc from ffigen
• Made all the AST classes extends AstNode, and implemented the visit and visitChildren methods.
  • This meant that BuiltInDeclaration had to be converted from an enum to a class
• Rewrote DependencyVisitor using the Visitation class

Ideally we'd share the AstNode, Visitor, and Visitation classes, but we don't really have a way of sharing internal code between native packages yet.

Part of #1358