Make creating AllAstNodeIter explicit.

This is to remove the ambiguity between calling iter on the specific structs like Document and calling iter on an AstNode by having an explicitly-named function to create the iterator.

src/types/mod.rs

@@ -13,7 +13,7 @@ pub use affiliated_keyword::AffiliatedKeyword;
 pub use affiliated_keyword::AffiliatedKeywordValue;
 pub use affiliated_keyword::AffiliatedKeywords;
 pub use affiliated_keyword::GetAffiliatedKeywords;
-pub(crate) use ast_node::AstNode;
+pub use ast_node::AstNode;
 pub use document::Document;
 pub use document::DocumentElement;
 pub use document::Heading;