Remove FeatureStructure._feats; dicts are ordered

I'm pretty sure the internal _feats list was just to ensure the order
of features so reserializations of AVMs would be in the same order.
This is no longer needed as dicts are now ordered in Python.

Author: goodmami

delphin/tdl.py

@@ -397,7 +397,6 @@ def __init__(self, values=None, docstring=None):
             tmplist = ConsList(values, end=cr)
             dl_list = _ImplicitAVM()
             dl_list._avm.update(tmplist._avm)
-            dl_list._feats = tmplist._feats
             self.last = 'LIST.' + tmplist._last_path
         else:
             dl_list = cr

delphin/tfs.py

@@ -36,17 +36,15 @@ class FeatureStructure:
             to feature values
     """
 
-    __slots__ = ('_avm', '_feats')
+    __slots__ = '_avm',
 
     _avm: FeatureDict
-    _feats: list[str]
 
     def __init__(
         self,
         featvals: Union[FeatureSeq, FeatureMap, None] = None,
     ) -> None:
         self._avm = {}
-        self._feats = []
         if featvals and hasattr(featvals, 'items'):
             featvals = list(featvals.items())
         for feat, val in list(featvals or []):
@@ -68,8 +66,6 @@ def __setitem__(self, key: str, val: Any) -> None:
         avm = self._avm
         subkeys = key.split('.', 1)
         subkey = subkeys[0].upper()
-        if subkey not in avm:
-            self._feats.append(subkey)
         if len(subkeys) == 1:
             avm[subkey] = val
         else:
@@ -142,12 +138,7 @@ def features(self, expand: bool = False) -> FeatureList:
         """
         fs = []
         if self._avm is not None:
-            if len(self._feats) == len(self._avm):
-                feats = self._feats
-            else:
-                feats = list(self._avm)
-            for feat in feats:
-                val = self._avm[feat]
+            for feat, val in self._avm.items():
                 if isinstance(val, FeatureStructure):
                     if not expand and val._is_notable():
                         fs.append((feat, val))