This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.

$(TOP) is defined in py/mkenv.mk and should be used to refer to the top
level of this repository.

There were several different spellings of MicroPython present in comments,
when there should be only one.

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

py/builtin.h declares mp_builtin_open with the first argument of type
size_t.  Make all implementations conform to this declaration.

It provides sys.stdin, sys.stdout, sys.stderr for bare-metal targets based
on mp_hal functions.

It can still be overwritten by a port in mpconfigport.h but for almost
all cases one can use the provided default.