Rename "rawsocket" module to "microsocket".

It's no longer intended to provide just "raw" socket interface, may include
some convenience methods for compatibility with CPython socket - but anyway
just minimal set required to deal with socket client and servers, not wider
network functionality.
io.File, socket types: Add fileno() method.

Implementation is duplicated, but tolerate this for now, because there's
no clear idea how to de-dup it.