Add support for storing args during an exception raised by an irq.

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
```
py: Make id() return small int for the most common address space mapping.

Many OSes/CPUs have affinity to put "user" data into lower half of address
space. Take advantage of that and remap such addresses into full small int
range (including negative part).

If address is from upper half, long int will be used. Previously, small
int was returned for lower quarter of address space, and upper quarter. For
2 middle quarters, long int was used, which is clearly worse schedule than
the above.
py: Implement __file__ attribute for modules.
Merge pull request #738 from dhylands/except-args

Add support for storing args during an exception raised by an irq.
py: Change lexer stream API to return bytes not chars.

Lexer is now 8-bit clean inside strings.
py: Add mp_obj_str_builder_end_with_len.

This allows to create str's with a smaller length than initially asked
for.
stmhal: Add USB_VCP class/object, for direct USB VCP control.

Before, pyb.stdin/pyb.stdout allowed some kind of access to the USB VCP
device, but it was basic access.

This patch adds a proper USB_VCP class and object with much more control
over the USB VCP device.  Create an object with pyb.USB_VCP(), then use
this object as if it were a UART object.  It has send, recv, read,
write, and other methods.  send and recv allow a timeout to be specified.

Addresses issue 774.
stmhal, accel: Increase start-up times to 30ms; add extra 30ms delay.

For accel to start-up reliably, need to wait 30ms between on/off, and
30ms for it to enter active mode.  With this fix the accel can be read
immediately after initialising it.

Addresses issue #763.
py: Make MP_OBJ_NEW_SMALL_INT cast arg to mp_int_t itself.

Addresses issue #724.
py: Improve handling of long-int overflow.

This removes mpz_as_int, since that was a terrible function (it
implemented saturating conversion).

Use mpz_as_int_checked and mpz_as_uint_checked.  These now work
correctly (they previously had wrong overflow checking, eg
print(chr(10000000000000)) on 32-bit machine would incorrectly convert
this large number to a small int).
Merge branch 'master' of https://github.com/micropython/micropython
py: Improve encoding scheme for line-number to bytecode map.

Reduces by about a factor of 10 on average the amount of RAM needed to
store the line-number to bytecode map in the bytecode prelude.

Using CPython3.4's stdlib for statistics: previously, an average of
13 bytes were used per (bytecode offset, line-number offset) pair, and
now with this improvement, that's down to 1.3 bytes on average.

Large RAM usage before was due to some very large steps in line numbers,
both from the start of the first line in a function way down in the
file, and also functions that have big comments and/or big strings in
them (both cases were significant).

Although the savings are large on average for the CPython stdlib, it
won't have such a big effect for small scripts used in embedded
programming.

Addresses issue #648.
README: Add USB VID/PID to dfu-util command.

This reflects how it's done in stmhal/Makefile, via deploy.
stmhal: Change 0:/ and 1:/ to /flash and /sd; add CWD support.

Some important changes to the way the file system is structured on the
pyboard:

1. 0: and 1: drive names are now replaced with POSIX inspired
directories, namely /flash and /sd.

2. Filesystem now supports the notion of a current working directory.
Supports the standard Python way of manipulating it: os.chdir and
os.getcwd.

3. On boot up, current directory is /flash if no SD inserted, else /sd
if SD inserted.  Then runs boot.py and main.py from the current dir.
This is the same as the old behaviour, but is much more consistent and
flexible (eg you can os.chdir in boot.py to change where main.py is run
from).

4. sys.path (for import) is now set to '' (current dir), plus /flash
and /flash/lib, and then /sd and /sd/lib if SD inserted.  This, along
with CWD, means that import now works properly.  You can import a file
from the current directory.

5. os.listdir is fixed to return just the basename, not the full path.

See issue #537 for background and discussion.