py/objstringio: Add += operator for BytesIO/StringIO.
py/objstringio: Add += operator for BytesIO/StringIO.

There's a common anti-pattern of using a string as an accumulating string
buffer, e.g.:

buf = ""
while ...:
    buf += "something"
use(buf)

This is highly inefficient. The right pattern is to use StringIO, but
this requires rewriting code to use .write() method:

buf = StringIO()
while ...:
    buf.write("something")
use(buf.getvalue())

Instead, by introducing implementation of __iadd__ (+=) method, effectively
making it an alias for .write(), it allows for very clear and concise code,
following the original code (and easily update existing 3rd-party code):

buf = StringIO()
while ...:
    buf += "something"
use(buf.getvalue())

Change-Id: I880e8a16d9f9628f5ef3ffa054aec5de8a66ca51
Signed-off-by: Paul Sokolovsky <9dc5061f178bb11813e14f1ed02ba27eaf21fa5d@users.sourceforge.net>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython