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.

This allows to create str's with a smaller length than initially asked
for.

Lexer is now 8-bit clean inside strings.

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

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.

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
```

Add GNUmakefile to the .gitignore file.

Addresses issue #765.

Allow DFU_UTIL to be overridden from the environment.

Enable 16-bit memory addresses for i2c.mem_read and i2c_mem_write

Deal with reading a buffer less than what was allocated.

to maintain compatibility with existing code.

With this fix, file_long_read now passes.

Currently broken for unicode input streams.

Recent changes to builtin print meant that print was printing to the
mp_sys_stdout_obj, which was sending data raw to the USB CDC device.
The data should be cooked so that \n turns into \r\n.

Fixes printing bugs introduced by
cb66f41e.

And not system printf(), like it was before. For this, move pfenv_printf()
from stmhal port to py/.

Add fsync for windows, i.e. _commit. See dce8876d

py: Add stream reading of n unicode chars; unicode support by default.

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.

Addresses issue #753.

Add teensy core files and use same toolchain as stmhal

Fix teensy to build on latest tree.