This patch adds a configuration option (MICROPY_CAN_OVERRIDE_BUILTINS)
which, when enabled, allows to override all names within the builtins
module.  A builtins override dict is created the first time the user
assigns to a name in the builtins model, and then that dict is searched
first on subsequent lookups.  Note that this implementation doesn't
allow deleting of names.

This patch also does some refactoring of builtins code, creating the
modbuiltins.c file.

Addresses issue #959.

This is useful if you need precise control over the speed of
peripherals (eg SPI clock).

The function is modeled after traceback.print_exception(), but unbloated,
and put into existing module to save overhead on adding another module.
Compliant traceback.print_exception() is intended to be implemented in
micropython-lib in terms of sys.print_exception().

This change required refactoring mp_obj_print_exception() to take pfenv_t
interface arguments.

Addresses #751.

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.

Uses DMA if interrupts are enabled, polling if they are disabled.

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.

mp_lexer_t type is exposed, mp_token_t type is removed, and simple lexer
functions (like checking current token kind) are now inlined.

This saves 784 bytes ROM on 32-bit unix, 348 bytes on stmhal, and 460
bytes on bare-arm.  It also saves a tiny bit of RAM since mp_lexer_t
is a bit smaller.  Also will run a bit more efficiently.

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.

Behaviour of array initialisation is subtly different for bytes,
bytearray and array.array when argument has buffer protocol.  This patch
gets us CPython conformant (except we allow initialisation of
array.array by buffer with length not a multiple of typecode).

This makes no change to the generated code, but it's now easier to
understand since unum is not a "global" variable anymore.

So can remove unnecessary stm32f4xx_hal_msp.c file.

TODO: Merge useful functionality from modpyb too.

This makes pyboard.py much more useful for long running scripts.  When
running a script via pyboard.py, it now waits until the script finishes,
with no timeout.  CTRL-C can be used to break out of the waiting if
needed.

pyb.delay and pyb.udelay now use systick if IRQs are enabled, otherwise
they use a busy loop.  Thus they work correctly when IRQs are disabled.
The busy loop is computed from the current CPU frequency, so works no
matter the CPU frequency.

By using the buffer protocol for these array operations, we now allow
addition of memoryview objects, and objects with "incompatible"
typecodes (in this case it just adds bytes naively).  This is an
extension to CPython which seems sensible.  It also reduces the code
size.

Addresses issue #994.

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

rodata can only go in iram/irom if it's accessed only using word loads
(ie no byte or half-word access).