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

We rely on the port setting and adjusting the stack size so there is
enough room to recover from hitting the stack limit.

The GIL macros are needed even if threading is not enabled.

This makes the VM/runtime thread safe, at the cost of not being able to
run code in parallel.

So the underlying thread implementation can do any necessary bookkeeping.

By using a single, global mutex, all memory-related functions (alloc,
free, realloc, collect, etc) are made thread safe.  This means that only
one thread can be in such a function at any one time.

Make dupterm subsystem close a term stream object when EOF or error occurs.
There's no other party than dupterm itself in a better position to do this,
and this is required to properly reclaim stream resources, especially if
multiple dupterm sessions may be established (e.g. as networking
connections).

Function to actually spool output terminal data to dupterm object.

That's just function which sets/gets dup terminal object, and can be
easily reused across ports.

Blanket wide to all .c and .h files.  Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.

Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.

As per issue #257, servo is better on TIM5 because TIM2 is connected to
more GPIO.

A big change.  Micro Python objects are allocated as individual structs
with the first element being a pointer to the type information (which
is itself an object).  This scheme follows CPython.  Much more flexible,
not necessarily slower, uses same heap memory, and can allocate objects
statically.

Also change name prefix, from py_ to mp_ (mp for Micro Python).