unix: Enable GC.

Conflicts:
	py/objstr.c

Also fix objstr.c to compile when floats disabled.

Also change formating modifier in test script (it still passes with
original format though).

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

Fail currently.

For example, we still build w/o GC enabled, so cannot really set heap size.

Previosuly just silently exited.

Very little has changed.  In Python 3.4 they removed the opcode
STORE_LOCALS, but in Micro Python we only ever used this for CPython
compatibility, so it was a trivial thing to remove.  It also allowed to
clean up some dead code (eg the 0xdeadbeef in class construction), and
now class builders use 1 less stack word.

Python 3.4.0 introduced the LOAD_CLASSDEREF opcode, which I have not
yet understood.  Still, all tests (apart from bytecode test) still pass.
Bytecode tests needs some more attention, but they are not that
important anymore.

I upgraded to Python 3.4.0, so needed to make these changes.  Hopefully
the tests still run with Python 3.3.x (the scripts use python3 so are
agnostic as to the subversion).

Bytecode tests are tightly coupled to the Python version, and now some
fail against Python 3.4.

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

Pfenv strn stack

Fix pyb.Led(n) code to work properly.

Fix INT_BUF_SIZE to work with 32-bit and 64-bit

Turn off full tests in string-format.py

Add some basic coverage tests