This cleans up vstr so that it's a pure "variable buffer", and the user
can decide whether they need to add a terminating null byte.  In most
places where vstr is used, the vstr did not need to be null terminated
and so this patch saves code size, a tiny bit of RAM, and makes vstr
usage more efficient.  When null termination is needed it must be
done explicitly using vstr_null_terminate.

accept might raise an exception, in which case the new socket is not
fully created.  It has a finaliser so will run close() method when GC'd.
Before this patch close would try to close an invalid socket.  Now
fixed.

setsockopt took address of stack value which became out of scope.  Now
fixed.

It needs to be scanned by GC.  Thanks to Daniel Campora.

With this patch str/bytes construction is streamlined.  Always use a
vstr to build a str/bytes object.  If the size is known beforehand then
use vstr_init_len to allocate only required memory.  Otherwise use
vstr_init and the vstr will grow as needed.  Then use
mp_obj_new_str_from_vstr to create a str/bytes object using the vstr
memory.

Saves code ROM: 68 bytes on stmhal, 108 bytes on bare-arm, and 336 bytes
on unix x64.

Remove include of stm32f4xx_hal.h, replace by include of MICROPY_HAL_H
where needed, and make it compile without float support.  This makes
these 3 modules much more generic and usable by other ports.

mp_obj_int_get_truncated is used as a "fast path" int accessor that
doesn't check for overflow and returns the int truncated to the machine
word size, ie mp_int_t.

Use mp_obj_int_get_truncated to fix struct.pack when packing maximum word
sized values.

Addresses issues #779 and #998.

This patch overhauls the network driver interface.  A generic NIC must
provide a set of C-level functions to implement low-level socket control
(eg socket, bind, connect, send, recv).  Doing this, the network and
usocket modules can then use such a NIC to implement proper socket
control at the Python level.

This patch also updates the CC3K and WIZNET5K drivers to conform to the
new interface, and fixes some bugs in the drivers.  They now work
reasonably well.

This is just a clean-up of the code.  Generated code is exactly the
same.

As per issue #876, the network module is used to configure NICs
(hardware modules) and configure routing.  The usocket module is
supposed to implement the normal Python socket module and selects the
underlying NIC using routing logic.

Right now the routing logic is brain dead: first-initialised,
first-used.  And the routing table is just a list of registered NICs.

cc3k and wiznet5k work, but not at the same time due to C name clashes
(to be fixed).

Note that the usocket module has alias socket, so that one can import
socket and it works as normal.  But you can also override socket with
your own module, using usocket at the backend.