Link references don't seem to work anymore.

MP_STATE_CTX was recently aliased to MP_STATE_THREAD and can now be
removed.

To improve maintainability of the code.

Previous to this patch, if the result of the round function overflowed a
small int, or was inf or nan, then a garbage value was returned.  With
this patch the correct big-int is returned if necessary and exceptions are
raised for inf or nan.

The C nearbyint function has exactly the semantics that Python's round()
requires, whereas C's round() requires extra steps to handle rounding of
numbers half way between integers.  So using nearbyint reduces code size
and potentially eliminates any source of errors in the handling of half-way
numbers.

Also, bare-metal implementations of nearbyint can be more efficient than
round, so further code size is saved (and efficiency improved).

nearbyint is provided in the C99 standard so it should be available on all
supported platforms.

One should test bigint, inf and nan to make sure all cases are covered.

Otherwise -0.0 is classified as a bigint, which for builds without bigints
will lead unexpectedly to an overflow.

Previous to this patch, if the result of the trunc/ceil/floor functions
overflowed a small int, or was inf or nan, then a garbage value was
returned.  With this patch the correct big-int is returned if necessary,
and exceptions are raised for inf or nan.

It improves readability of code and reduces the chance to make a mistake.

This patch also fixes a bug with nan-boxing builds by rounding up the
calculation of the new NSLOTS variable, giving the correct number of slots
(being 4) even if mp_obj_t is larger than the native machine size.

These values are used to compute memory addresses and so size_t is the
more appropriate type to use.

This assures after cleaning all build artefacts (qstr related files,
generated version header) have been removed.

Instead of having the PlatformToolset property hardcoded to a specific
version just set it to the value of DefaultPlatformToolset: this gets
defined according to the commandline environment in which the build was
started.
Instead of just supporting VS2015 the project can now be built by any
version from VS2013 to VS2017 and normally future versions as well, without
quirks like VS asking whether you want to upgrade the project to the latest
version (as was the case when opening the project in VS2017) or not being
able to build at all (as was the case when opening the project in VS2013).

Also adjust the .gitignore file to ignore any artefacts from VS2017.

The -ansi flag is used for C dialect selection and it is equivalent to -std=c90.
Because it goes right before -std=gnu99 it is ignored as for conflicting flags
GCC always uses the last one.

Now, passing a keyword argument that is not expected will correctly report
that fact.  If normal or detailed error messages are enabled then the name
of the unexpected argument will be reported.

This patch decreases the code size of bare-arm and stmhal by 12 bytes, and
cc3200 by 8 bytes.  Other ports (minimal, unix, esp8266) remain the same in
code size.  For terse error message configuration this is because the new
message is shorter than the old one.  For normal (and detailed) error
message configuration this is because the new error message already exists
in py/objnamedtuple.c so there's no extra space in ROM needed for the
string.

GIL behaviour should be handled by the port.  And ports probably want to
define sleep_us so that it doesn't release the GIL, to improve timing
accuracy.

When locking/unlocking a mutex we only need to protect against a thread
switch, not general interrupts.

This way, Timer and ExtInt callbacks can interrupt the low-priority
pendsv handler (for example thread switching).

Thanks to @robert-hh.

We can actually handle interrupts during a thread switch (because we always
have a valid stack), but only if those interrupts don't access any of the
thread state (because the state may not correspond to the stack pointer).
So to be on the safe side we disable interrupts during the very short
period of the thread state+stack switch.

The scheduler being locked general means we are running a scheduled
function, and switching to another thread violates that, so don't switch in
such a case (even though we technically could).

And if we are running a scheduled function then we want to finish it ASAP,
so we shouldn't switch to another thread.

Furthermore, ports with threading enabled will lock the scheduler during a
hard IRQ, and this patch to the VM will make sure that threads are not
switched during a hard IRQ (which would crash the VM).

MHLSB and MHMSB formats are added to the framebuf module, which have 8
adjacent horizontal pixels represented in a single byte.

Since eaef6b53 writes are used instead of
reads.