Add async support

This allows for async functions to be passed to the Flask class
instance, for example as a view function,

    @app.route("/")
    async def index():
        return "Async hello"

this comes with a cost though of poorer performance than using the
sync equivalent.

asgiref is the standard way to run async code within a sync context,
and is used in Django making it a safe and sane choice for this.

src/flask/scaffold.py

@@ -4,6 +4,7 @@ import pkgutil
 import sys
 from collections import defaultdict
 from functools import update_wrapper
+from inspect import iscoroutinefunction
 
 from jinja2 import FileSystemLoader
 from werkzeug.exceptions import default_exceptions
@@ -12,6 +13,7 @@ from werkzeug.exceptions import HTTPException
 from .cli import AppGroup
 from .globals import current_app
 from .helpers import locked_cached_property
+from .helpers import run_async
 from .helpers import send_from_directory
 from .templating import _default_template_ctx_processor
 
@@ -484,7 +486,7 @@ class Scaffold:
         """
 
         def decorator(f):
-            self.view_functions[endpoint] = f
+            self.view_functions[endpoint] = self.ensure_sync(f)
             return f
 
         return decorator
@@ -508,7 +510,7 @@ class Scaffold:
         return value from the view, and further request handling is
         stopped.
         """
-        self.before_request_funcs[None].append(f)
+        self.before_request_funcs.setdefault(None, []).append(self.ensure_sync(f))
         return f
 
     @setupmethod
@@ -524,7 +526,7 @@ class Scaffold:
         should not be used for actions that must execute, such as to
         close resources. Use :meth:`teardown_request` for that.
         """
-        self.after_request_funcs[None].append(f)
+        self.after_request_funcs.setdefault(None, []).append(self.ensure_sync(f))
         return f
 
     @setupmethod
@@ -563,7 +565,7 @@ class Scaffold:
            debugger can still access it.  This behavior can be controlled
            by the ``PRESERVE_CONTEXT_ON_EXCEPTION`` configuration variable.
         """
-        self.teardown_request_funcs[None].append(f)
+        self.teardown_request_funcs.setdefault(None, []).append(self.ensure_sync(f))
         return f
 
     @setupmethod
@@ -659,7 +661,7 @@ class Scaffold:
                 " instead."
             )
 
-        self.error_handler_spec[None][code][exc_class] = f
+        self.error_handler_spec[None][code][exc_class] = self.ensure_sync(f)
 
     @staticmethod
     def _get_exc_class_and_code(exc_class_or_code):
@@ -684,6 +686,19 @@ class Scaffold:
         else:
             return exc_class, None
 
+    def ensure_sync(self, func):
+        """Ensure that the returned function is sync and calls the async func.
+
+        .. versionadded:: 2.0
+
+        Override if you wish to change how asynchronous functions are
+        run.
+        """
+        if iscoroutinefunction(func):
+            return run_async(func)
+        else:
+            return func
+
 
 def _endpoint_from_view_func(view_func):
     """Internal helper that returns the default endpoint for a given