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

Enhance str.format support

This adds support for almost everything (the comma isn't currently
supported).

The "unspecified" type with floats also doesn't behave exactly like
python.

Tested under unix with float and double
Spot tested on stmhal

It's not completely satisfactory, because a failed call to __getattr__
should not raise an exception.

__setattr__ could be implemented, but it would slow down all stores to a
user created object.  Need to implement some caching system.

Because it's superfluos in the presence of type(), a remenant from Python's
"old classes".

Because it's runtime reflection feature, not required for many apps.
Rant time:
Python could really use better str() vs repr() distinction, for example,
repr(type) could be "<class 'foo'>" (as it is now), and str(type) just
"foo". But alas, getting straight name requires adhoc attribute.

Update README.md

Don't store final, failing value to the loop variable.  This fix also
makes for .. range a bit more efficient, as it uses less store/load
pairs for the loop variable.

Yes, I know, a good compiler will optimise this away, but I feel this is
neater.

It's virtual because it's not shown in dir(...). (That's also how CPython
has it).

There was thinkos that either send_value or throw_value is specified, but
there were cases with both. Note that send_value is pushed onto generator's
stack - but that's probably only good, because if we throw exception into
gen, it should not ever use send_value, and that will be just extra "assert".

In this case, the exception is just re-thrown - the ideas is that object
doesn't handle this exception specially, so it will propagated per Python
semantics.

.throw() propagates any exceptions, and .close() swallows them. Yielding
in reponse to .throw(GeneratorExit) is still fatal, and we need to
handle it for .throw() case separately (previously it was handled only
for .close() case).

Obscure corner cases due to test_pep380.py.