Fix crash when passing alot of defaults
Merge pull request #110 from undingen/defaults_crash

Fix crash when passing alot of defaults
let's only clear these in debug mode
Fix a bug when CFG-izing certain structures in try blocks

Augassigns and for loops had an issue where they would try to
do something like 'i = i.__iadd__(j)', ie have multiple operations
in a single statement.  If an exception occurs, it's not clear if
the name got redefined; so far we just assume that it always does (since
we don't have the mechanism to propagate the name along only one
control flow edge).

So we have to split up those multi-operation-statements into multiple
statements.
Add int->long promotion on overflow

This commit is a straightforward implementation that doesn't include
any speculation-like optimizations.  For operations that can overflow,
just relax the type-return-specification to UNKNOWN, and do the overflow
checks in the runtime.  This means that we no longer emit fast native
integer instructions even if we know operands are ints.

Will have to add optimizations:
- range analysis so that we can know there won't be overflow
- deopt-on-overflow so that we can work with unboxed ints even if there's potential overflow
Implement function decorators
Support decorators on classes as well
Implement set literals
Implement generator expressions by converting to generator functions