Improve code completion performance (CompletionsMenuControl)

When there are many completions, compute the width of the "meta" column in the
completion menu based on the first 200 completions, otherwise this can be very
slow.

Author: jonathanslenders

src/prompt_toolkit/layout/menus.py

@@ -164,9 +164,13 @@ def meta_width(completion: Completion) -> int:
             return get_cwidth(completion.display_meta_text)
 
         if self._show_meta(complete_state):
-            return min(
-                max_width, max(meta_width(c) for c in complete_state.completions) + 2
-            )
+            # If the amount of completions is over 200, compute the width based
+            # on the first 200 completions, otherwise this can be very slow.
+            completions = complete_state.completions
+            if len(completions) > 200:
+                completions = completions[:200]
+
+            return min(max_width, max(meta_width(c) for c in completions) + 2)
         else:
             return 0