Arguments of an unknown type cannot be skipped and continuing to parse a
format string after encountering an unknown format specifier leads to
undefined behaviour.  This patch helps to find use of unsupported formats.

With backwards compatibility for pyboard.

The idea is that all ports can use these helper methods and only need to
provide initialisation of the SPI bus, as well as a single transfer
function.  The coding pattern follows the stream protocol and helper
methods.

This extra forward slash for the starting-point directory is unnecessary
and leads to additional slashes on Max OS X which mean that the frozen
files cannot be imported.

Fixes #2374.

Or alternatively, refer to an exact library file, not just phony target
"lib".

And remove reference to deprecated pyb.delay() and pyb.millis().

This allows 1-wire drivers (eg DS18X20) to perform in-place operations and
hence do less memory allocations.

The OneWire class is now in its own onewire.py module, and the temperature
sensor class is in its own ds18x20.py module.  The latter is renamed to
DS18X20 to reflect the fact that it will support both the "S" and "B"
variants of the device.

These files are moved to the modules/ subdirectory to take advantage of
frozen bytecode.

Use the `SPI` factory function in the examples, and use
proper baud rate of 80 000 000.

It has reliability issues (cause unknown at this time).

This rename was missed in the previous patch.

Also at _t to mp_exc_stack pre-declaration in struct typedef.

Try to un-mount a file system and re-mount it again.

Saves a few bytes of code size.

As per CPython.

This patch makes second and next calls to <socket>.close() a no-op.
It prevents GC from closing the underlying resource after user
already used <socket>.close() explicitly.

fixes #2355

There can be stray pointers in memory blocks that are not properly zero'd
after allocation.  This patch adds a new config option to always zero all
allocated memory (via gc_alloc and gc_realloc) and hence help to eliminate
stray pointers.

See issue #2195.

This is an object-oriented approach, where uos is only a proxy for the
methods on the vfs object.  Some internals had to be exposed (the STATIC
keyword removed) for this to work.

Fixes #2338.