uasyncio: awrite: Use 3-arg .write(), accept offset/size too.

Use MicroPython .write() extension of passing offset/size to efficiently
spool buffers larger than socket output buffer. Also, make awrite()
accept these params too.

uasyncio/uasyncio/__init__.py

@@ -131,17 +131,18 @@ class StreamWriter:
         self.s = s
         self.extra = extra
 
-    def awrite(self, buf):
+    def awrite(self, buf, off=0, sz=-1):
         # This method is called awrite (async write) to not proliferate
         # incompatibility with original asyncio. Unlike original asyncio
         # whose .write() method is both not a coroutine and guaranteed
         # to return immediately (which means it has to buffer all the
         # data), this method is a coroutine.
-        sz = len(buf)
+        if sz == -1:
+            sz = len(buf) - off
         if DEBUG and __debug__:
             log.debug("StreamWriter.awrite(): spooling %d bytes", sz)
         while True:
-            res = self.s.write(buf)
+            res = self.s.write(buf, off, sz)
             # If we spooled everything, return immediately
             if res == sz:
                 if DEBUG and __debug__:
@@ -152,7 +153,7 @@ class StreamWriter:
             if DEBUG and __debug__:
                 log.debug("StreamWriter.awrite(): spooled partial %d bytes", res)
             assert res < sz
-            buf = buf[res:]
+            off += res
             sz -= res
             yield IOWrite(self.s)
             #assert s2.fileno() == self.s.fileno()