Migrate tnode implementation to tnode.cpp

Also improve commenting.

CMakeLists.txt

@@ -50,7 +50,7 @@ SET(FISH_SRCS
     src/parse_execution.cpp src/parse_productions.cpp src/parse_tree.cpp
     src/parse_util.cpp src/parser.cpp src/parser_keywords.cpp src/path.cpp
     src/postfork.cpp src/proc.cpp src/reader.cpp src/sanity.cpp src/screen.cpp
-    src/signal.cpp src/tokenizer.cpp src/utf8.cpp src/util.cpp
+    src/signal.cpp src/tnode.cpp src/tokenizer.cpp src/utf8.cpp src/util.cpp
     src/wcstringutil.cpp src/wgetopt.cpp src/wildcard.cpp src/wutil.cpp
 )