uasyncio.websocket.server: Websocket server implementation for uasyncio.

During development, following questions were posed, and subsequently,
answered:

Q #1: Should this be in uasyncio package at all? Upstream doesn't have
this. Pro: will be easier for people do discover (see e.g.
https://github.com/micropython/micropython-lib/issues/148)

A: uasyncio diverges more and more from asyncio, so if something is
convinient for uasyncio, there's no need to look back at asyncio.

Q #2: This provides implements 2 ways to create a WS connections:
1) using start_ws_server(); 2) using wrapping existing StreamReader
and StreamWriter. History: initial prototype of course used 2). But
the idea was "it should be like the official start_server()!!1". But
then I though how to integrate it e.g. with Picoweb, and became clear
that 2) is the most flixble way. So, 1) is intended to be removed.

A: 1) was removed and is not part of the merged version of the patch.

Q #3: Uses native websocket module for read path, but has own
write path due to https://github.com/micropython/micropython/issues/3396

A: So far, so good.

Q #4: Requires https://github.com/micropython/micropython-lib/pull/227
due to https://github.com/micropython/micropython/issues/3394 .

A: The prerequisite was merged.

uasyncio.websocket.server/uasyncio/websocket/server.py

@@ -0,0 +1,63 @@
+import uasyncio
+import uhashlib, ubinascii
+import websocket
+
+
+def make_respkey(webkey):
+    d = uhashlib.sha1(webkey)
+    d.update(b"258EAFA5-E914-47DA-95CA-C5AB0DC85B11")
+    respkey = d.digest()
+    respkey = ubinascii.b2a_base64(respkey) #[:-1]
+    # Return with trailing "\n".
+    return respkey
+
+
+class WSWriter:
+
+    def __init__(self, reader, writer):
+        # Reader is passed for symmetry with WSReader() and ignored.
+        self.s = writer
+
+    async def awrite(self, data):
+        assert len(data) < 126
+        await self.s.awrite(b"\x81")
+        await self.s.awrite(bytes([len(data)]))
+        await self.s.awrite(data)
+
+
+def WSReader(reader, writer):
+
+        webkey = None
+        while 1:
+            l = yield from reader.readline()
+            print(l)
+            if not l:
+                raise ValueError()
+            if l == b"\r\n":
+                break
+            if l.startswith(b'Sec-WebSocket-Key'):
+                webkey = l.split(b":", 1)[1]
+                webkey = webkey.strip()
+
+        if not webkey:
+            raise ValueError("Not a websocker request")
+
+        respkey = make_respkey(webkey)
+
+        await writer.awrite(b"""\
+HTTP/1.1 101 Switching Protocols\r
+Upgrade: websocket\r
+Connection: Upgrade\r
+Sec-WebSocket-Accept: """)
+        await writer.awrite(respkey)
+        # This will lead to "<key>\n\r\n" being written. Not exactly
+        # "\r\n\r\n", but browsers seem to eat it.
+        await writer.awrite("\r\n")
+        #await writer.awrite("\r\n\r\n")
+
+        print("Finished webrepl handshake")
+
+        ws = websocket.websocket(reader.ios)
+        rws = uasyncio.StreamReader(reader.ios, ws)
+
+        return rws