Actually memoize previous_part and next_part.

This improves performance further and doesn't really affect peak memory
consumption.

Because of eng/libadalang/langkit#710, overriden properties lose their
memoization attribute. So, this change inlines the logic of the
overriding properties inside the base properties to make sure
memoization is applied.

ada/ast.py

@@ -6921,12 +6921,18 @@ def previous_part(go_to_incomplete=(Bool, True)):
         """
         Returns the previous part for this type decl.
         """
-        return If(
+        return Cond(
             Self.is_generic_formal,
 
             # A generic formal type never has a previous part
             No(T.BaseTypeDecl.entity),
 
+            Self.is_a(ClasswideTypeDecl),
+            Entity.cast(ClasswideTypeDecl).type_decl
+            .previous_part(go_to_incomplete).then(
+                lambda pp: pp.classwide_type
+            ),
+
             # Otherwise look for the previous part in the immediate enclosing
             # declarative region.
             Self.name.then(
@@ -6976,6 +6982,18 @@ def next_part():
                 )
             )),
 
+            lambda cwt=T.ClasswideTypeDecl: Let(
+                lambda td=cwt.type_decl: td.next_part.then(
+                    # Sometimes `next_part` returns Entity itself, so check
+                    # that to avoid an infinite loop.
+                    lambda np: If(
+                        td == np,
+                        cwt,
+                        np.classwide_type
+                    )
+                )
+            ),
+
             lambda _: If(
                 Entity.is_private
                 & Not(Entity.is_generic_formal),
@@ -7384,26 +7402,6 @@ def discriminants_list(
     ):
         return Entity.type_decl.discriminants_list(stop_recurse_at)
 
-    @langkit_property(public=True, return_type=T.BaseTypeDecl.entity,
-                      memoized=True)
-    def previous_part(go_to_incomplete=(Bool, True)):
-        return Entity.type_decl.previous_part(go_to_incomplete).then(
-            lambda pp: pp.classwide_type
-        )
-
-    @langkit_property()
-    def next_part():
-        td = Var(Entity.type_decl)
-        return td.next_part.then(
-            # Sometimes `next_part` returns Entity itself, so check that
-            # to avoid an infinite loop.
-            lambda np: If(
-                td == np,
-                Entity,
-                np.classwide_type
-            )
-        )
-
     canonical_type = Property(Entity.type_decl.canonical_type.then(
         # The canonical type should be classwide whenever it makes sense (e.g.
         # if the canonical type is a tagged record type.) Otherwise return