Simplify the async handling code

Firstly `run_sync` was a misleading name as it didn't run anything,
instead I think `async_to_sync` is much clearer as it converts a
coroutine function to a function. (Name stolen from asgiref).

Secondly trying to run the ensure_sync during registration made the
code more complex and brittle, e.g. the _flask_async_wrapper
usage. This was done to pay any setup costs during registration rather
than runtime, however this only saved a iscoroutne check. It allows
the weirdness of the Blueprint and Scaffold ensure_sync methods to be
removed.

Switching to runtime ensure_sync usage provides a method for
extensions to also support async, as now documented.

tests/test_async.py

@@ -6,7 +6,7 @@ import pytest
 from flask import Blueprint
 from flask import Flask
 from flask import request
-from flask.helpers import run_async
+from flask.helpers import async_to_sync
 
 pytest.importorskip("asgiref")
 
@@ -137,4 +137,4 @@ def test_async_before_after_request():
 @pytest.mark.skipif(sys.version_info >= (3, 7), reason="should only raise Python < 3.7")
 def test_async_runtime_error():
     with pytest.raises(RuntimeError):
-        run_async(None)
+        async_to_sync(None)