render_markdown_path: Convert dicts to dict item tuples for caching.

Calls to `render_markdown_path` weren't getting cached since the context
argument is unhashable, and the `ignore_unhashable_lru_cache` decorator ignores
such calls. This commit adds a couple of more decorators - one which converts
dict arguments to the function to a dict items tuple, and another which converts
dict items tuple arguments back to dicts. These two decorators used along with
the `ignore_unhashable_lru_cache` decorator ensure that the calls to
`render_markdown_path` with the context dict argument are also cached.

The time to run zerver.tests.test_urls.PublicURLTest.test_public_urls drops by
about 50% from 8.4s to 4.1s with this commit. The time to run
zerver.tests.test_docs.DocPageTest.test_doc_endpoints drops by about 20% from
3.2s to 2.5s.

zerver/lib/cache.py

@@ -534,3 +534,33 @@ def ignore_unhashable_lru_cache(maxsize: int=128, typed: bool=False) -> DECORATO
         return wrapper
 
     return decorator
+
+def dict_to_items_tuple(user_function: Callable[..., Any]) -> Callable[..., Any]:
+    """Wrapper that converts any dict args to dict item tuples."""
+    def dict_to_tuple(arg: Any) -> Any:
+        if isinstance(arg, dict):
+            return tuple(sorted(arg.items()))
+        return arg
+
+    def wrapper(*args: Any, **kwargs: Any) -> Any:
+        new_args = (dict_to_tuple(arg) for arg in args)
+        return user_function(*new_args, **kwargs)
+
+    return wrapper
+
+def items_tuple_to_dict(user_function: Callable[..., Any]) -> Callable[..., Any]:
+    """Wrapper that converts any dict items tuple args to dicts."""
+    def dict_items_to_dict(arg: Any) -> Any:
+        if isinstance(arg, tuple):
+            try:
+                return dict(arg)
+            except TypeError:
+                pass
+        return arg
+
+    def wrapper(*args: Any, **kwargs: Any) -> Any:
+        new_args = (dict_items_to_dict(arg) for arg in args)
+        new_kwargs = {key: dict_items_to_dict(val) for key, val in kwargs.items()}
+        return user_function(*new_args, **new_kwargs)
+
+    return wrapper