Before this patch raising a big-int to a negative power would just return
0.  Now it returns a floating-point number with the correct value.

It's more efficient as an inline function, and saves code size.

Before this patch the mperrno.h file could be included and would silently
succeed with incorrect config settings, because mpconfig.h was not yet
included.

Otherwise they can interfere (eg redefinition of "abort") with other source
files in a given uPy port.

Without this cast the print will give a wrong result on nan-boxing builds.

If constants (eg mp_const_none_obj) are placed in very high memory
locations that require 64-bits for the pointer then the assembler must be
able to emit instructions to move such pointers to one of the top 8
registers (ie r8-r15).

It's not used anywhere else in the VM loop, and clashes with (is shadowed
by) the n_state variable that's redeclared towards the end of the
mp_execute_bytecode function.  Code size is unchanged.

The code conventions suggest using header guards, but do not define how
those should look like and instead point to existing files. However, not
all existing files follow the same scheme, sometimes omitting header guards
altogether, sometimes using non-standard names, making it easy to
accidentally pick a "wrong" example.

This commit ensures that all header files of the MicroPython project (that
were not simply copied from somewhere else) follow the same pattern, that
was already present in the majority of files, especially in the py folder.

The rules are as follows.

Naming convention:
* start with the words MICROPY_INCLUDED
* contain the full path to the file
* replace special characters with _

In addition, there are no empty lines before #ifndef, between #ifndef and
one empty line before #endif. #endif is followed by a comment containing
the name of the guard macro.

py/grammar.h cannot use header guards by design, since it has to be
included multiple times in a single C file. Several other files also do not
need header guards as they are only used internally and guaranteed to be
included only once:
* MICROPY_MPHALPORT_H
* mpconfigboard.h
* mpconfigport.h
* mpthreadport.h
* pin_defs_*.h
* qstrdefs*.h

gc_free() expects either NULL or a valid pointer into the heap, so the
checks for a valid pointer can be turned into assertions.

Adds nothing to the code size, since it uses existing empty slots in the
type structures.

Reduces code size.

This refactoring saves code space.

Code size change in bytes for this patch is:

   bare-arm:   -72
minimal x86:   -48
   unix x64:   -32
unix nanbox:  -120
     stmhal:   -68
     cc3200:   -64
    esp8266:   -56

The line in docs/conf.py with the release/version number was recently
changed and this patch makes the makeversionhdr.py script work again.

This matches the coding/layout style of all the other objects.

Prior to this patch there were 2 paths for creating the namedtuple, one for
when no keyword args were passed, and one when there were keyword args.
And alloca was used in the keyword-arg path to temporarily create the array
of elements for the namedtuple, which would then be copied to a
heap-allocated object (the namedtuple itself).

This patch simplifies the code by combining the no-keyword and keyword
paths, and removing the need for the alloca by constructing the namedtuple
on the heap before populating it.

Heap usage in unchanged, stack usage is reduced, use of alloca is removed,
and code size is not increased and is actually reduced by between 20-30
bytes for most ports.

The while-loop that calls chop_component will guarantee that level==-1 at
the end of the loop.  Hence the code following it is unnecessary.

The check for p==this_name will catch imports that are beyond the
top-level, and also covers the case of new_mod_q==MP_QSTR_ (equivalent to
new_mod_l==0) so that check is removed.

There is also a new check at the start for level>=0 to guard against
__import__ being called with bad level values.

Previous to this patch, a label with value "0" was used to indicate an
invalid label, but that meant a wasted word (at slot 0) in the array of
label offsets.  This patch adjusts the label indices so the first one
starts at 0, and the maximum value indicates an invalid label.

This patch fixes a bug whereby the Python stack was not correctly reset if
there was a break/continue statement in the else black of an optimised
for-range loop.

For example, in the following code the "j" variable from the inner for loop
was not being popped off the Python stack:

    for i in range(4):
        for j in range(4):
            pass
        else:
            continue

This is now fixed with this patch.

In CPython 3.4 this raises a SyntaxError.  In CPython 3.5+ having a
positional after * is allowed but uPy has the wrong semantics and passes
the arguments in the incorrect order.  To prevent incorrect use of a
function going unnoticed it is important to raise the SyntaxError in uPy,
until the behaviour is fixed to follow CPython 3.5+.