py/modio: Add uio.IOBase class to allow to define user streams.

A user class derived from IOBase and implementing readinto/write/ioctl can
now be used anywhere a native stream object is accepted.

The mapping from C to Python is:

    stream_p->read  --> readinto(buf)
    stream_p->write --> write(buf)
    stream_p->ioctl --> ioctl(request, arg)

Among other things it allows the user to:

- create an object which can be passed as the file argument to print:
  print(..., file=myobj), and then print will pass all the data to the
  object via the objects write method (same as CPython)
- pass a user object to uio.BufferedWriter to buffer the writes (same as
  CPython)
- use select.select on a user object
- register user objects with select.poll, in particular so user objects can
  be used with uasyncio
- create user files that can be returned from user filesystems, and import
  can import scripts from these user files

For example:

    class MyOut(io.IOBase):
        def write(self, buf):
            print('write', repr(buf))
            return len(buf)

    print('hello', file=MyOut())

The feature is enabled via MICROPY_PY_IO_IOBASE which is disabled by
default.

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