stream_read(): Shrink memory block to actual read size.
unix socket: Add send() and recv() methods.

CPython _socket actually have only those and doesn't provide stream
interface (higher-level CPython "socket" what adds this).

+516 bytes x86.
Don't preimport socket module.
Add dummy bytes() constructor.

Currently, MicroPython strings are mix between CPython byte and unicode
strings. So, conversion is null so far. This dummy implementation is
intended for compatibility with CPython (so, same code can run on both).
socket-client, socket-server examples now run on both Micro- and C-Python.