vlib: add toml module + tests (#11964)

vlib/toml/ast/walker/walker.v

@@ -0,0 +1,37 @@
+module walker
+
+import toml.ast
+
+// Visitor defines a visit method which is invoked by the walker in each node it encounters.
+pub interface Visitor {
+	visit(node &ast.Node) ?
+}
+
+pub type InspectorFn = fn (node &ast.Node, data voidptr) ?
+
+struct Inspector {
+	inspector_callback InspectorFn
+mut:
+	data voidptr
+}
+
+pub fn (i &Inspector) visit(node &ast.Node) ? {
+	i.inspector_callback(node, i.data) or { return err }
+}
+
+// inspect traverses and checks the AST node on a depth-first order and based on the data given
+pub fn inspect(node &ast.Node, data voidptr, inspector_callback InspectorFn) ? {
+	walk(Inspector{inspector_callback, data}, node) ?
+}
+
+// walk traverses the AST using the given visitor
+pub fn walk(visitor Visitor, node &ast.Node) ? {
+	if node is map[string]ast.Node {
+		n := node as map[string]ast.Node
+		for _, nn in n {
+			walk(visitor, &nn) ?
+		}
+	} else {
+		visitor.visit(node) ?
+	}
+}