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.

tests/test_async.py

@@ -0,0 +1,33 @@
+import asyncio
+
+import pytest
+
+from flask import abort
+from flask import Flask
+from flask import request
+
+
+@pytest.fixture(name="async_app")
+def _async_app():
+    app = Flask(__name__)
+
+    @app.route("/", methods=["GET", "POST"])
+    async def index():
+        await asyncio.sleep(0)
+        return request.method
+
+    @app.route("/error")
+    async def error():
+        abort(412)
+
+    return app
+
+
+def test_async_request_context(async_app):
+    test_client = async_app.test_client()
+    response = test_client.get("/")
+    assert b"GET" in response.get_data()
+    response = test_client.post("/")
+    assert b"POST" in response.get_data()
+    response = test_client.get("/error")
+    assert response.status_code == 412