Rename AST structures to Tree and relocate freeing logic

v0/ast.h

@@ -8,4 +8,14 @@
 #include "ast/declaration.h"
 #include "ast/module.h"
 
+/**
+ * Frees a module and all its children.
+ */
+void ast_free_module(ModuleTree* module);
+
+/**
+ * Frees a type expression.
+ */
+void ast_free_type(TypeTree* type);
+
 #endif