The user code should call micropython.alloc_emergency_exception_buf(size)
where size is the size of the buffer used to print the argument
passed to the exception.

With the test code from #732, and a call to
micropython.alloc_emergenncy_exception_buf(100) the following error is
now printed:
```python
>>> import heartbeat_irq
Uncaught exception in Timer(4) interrupt handler
Traceback (most recent call last):
  File "0://heartbeat_irq.py", line 14, in heartbeat_cb
NameError: name 'led' is not defined
```

With unicode enabled, this patch allows reading a fixed number of
characters from text-mode streams; eg file.read(5) will read 5 unicode
chars, which can made of more than 5 bytes.

For an ASCII stream (ie no chars > 127) it only needs to do 1 read.  If
there are lots of non-ASCII chars in a stream, then it needs multiple
reads of the underlying object.

Adds a new test for this case.  Enables unicode support by default on
unix and stmhal ports.

emitnative in particular requires nlr_* to be real functions, so doesn't
compile with MICROPY_NLR_SETJMP=1.

But much smaller and memory-efficient. Uses Python builtin data structures
(dict, tuple, int) to describe structure layout.

Implementing it as a static constant is a bit peculiar and require cooperation
from long int implementation.

See discussion in issue #50.

Unicode is disabled by default for now, since FileIO.read(n) is
currently not implemented for text-mode files, and this is an
often function.

Ports which wants to have it, should define MICROPY_PY_SYS_PLATFORM to a
string value they need.

- Move the includes for alloca() intp mpconfigport.h

As I suspected for a long time, for x86, register helper doesn't really make
any difference - there's simply not enough register to keep anything in
them for any prolonged time. Anything gets pushed on stack anyway. So, on
x86, uPy passed all tests even with empty reg helper. So, this setjmp
implementation goes as "untested".

This renames:
MICROPY_PY_FROZENSET -> MICROPY_PY_BUILTINS_FROZENSET
MICROPY_PY_PROPERTY -> MICROPY_PY_BUILTINS_PROPERTY
MICROPY_PY_SLICE -> MICROPY_PY_BUILTINS_SLICE
MICROPY_ENABLE_FLOAT -> MICROPY_PY_BUILTINS_FLOAT

See issue #35 for discussion.

Now of the form MICROPY_PY_*.  See issue #35.

MP_ALLOC_* -> MICROPY_ALLOC_*
MICROPY_PATH_MAX -> MICROPY_ALLOC_PATH_MAX
MICROPY_ENABLE_REPL_HELPERS -> MICROPY_HELPER_REPL
MICROPY_ENABLE_LEXER_UNIX -> MICROPY_HELPER_LEXER_UNIX
MICROPY_EXTRA_* -> MICROPY_PORT_*

See issue #35.

io.FileIO is binary I/O, ans actually optional. Default file type is
io.TextIOWrapper, which provides str results. CPython3 explicitly describes
io.TextIOWrapper as buffered I/O, but we don't have buffering support yet
anyway.

stat() is bad function to use using FFI, because its ABI is largely private.
To start with, Glibc .so doesn't even have "stat" symbol. Then, layout of
struct stat is too implementation-dependent. So, introduce _os to deal
with stat() and other similar cases.

When disabling these via mpconfigport.mk or on the commandline,
the correspoding build options are not set and the sources are not
built so the modules should not be added to the
MICROPY_EXTRA_BUILTIN_MODULES list since they are undefined.

For consistency with MICROPY_MOD_SYS_STDFILES, etc.

Takes 416 text bytes on x86.

Enable only on unix. To avoid unpleasant surprises with error codes.

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/.

On stmhal, computed gotos make the binary about 1k bigger, but makes it
run faster, and we have the room, so why not.  All tests pass on
pyboard using computed gotos.

Not all functions implemented.  Not enabled on pyboard.

Disabled by default.  Enabled in unix port.

Enabled by MICROPY_ENABLE_PROPERTY.

This is to reduce ROM usage.  stream_p is used in file and socket types
only (at the moment), so seems a good idea to make the protocol
functions a pointer instead of the actual structure.

It saves 308 bytes of ROM in the stmhal/ port, 928 in unix/.

Finishes addressing issue #424.

In the end this was a very neat refactor that now makes things a lot
more consistent across the py code base.  It allowed some
simplifications in certain places, now that everything is a dict object.

Also converted builtins tables to dictionaries.  This will be useful
when we need to turn builtins into a proper module.

GC support for "unix" port was implemented some time ago, but still not
enabled.