Types:

• SyntaxLayout<T>: represents the layout of the T syntax layout type
• SyntaxLayoutProperty: represents an element in SyntaxLayout. like AnyKeyPath
• ConcreteSyntaxProperty<Base, Value>: represent a typed property. like KeyPath<Root, Value>

API:

• _LeafSyntaxNodeProtocol.syntaxKind to get the syntax kind from the meta type.
• _LayoutSyntaxNodeProtocol protocol for non-collection leaf syntax nodes
• _LayoutSyntaxNodeProtocol.layout to get the layout (SyntaxLayout<Self>)

Usage:

func usage(node: some SyntaxProtocol) {
  node.propertyInParent == FuncDeclSyntax.layout[.funcKeyword]
}

This is faster than keyPathInParent and comparing KeyPath. Combined with #2842, BasicFormat performs 30% faster