Drop redundant ref-counting on parallel swap assignments

[scoder]

The ref-counting for parallel swap assignments is redundant, i.e.

cdef object a = ..., b = ...
a,b = b,a

should be a simple pointer exchange through a temp variable. The same applies to swapping list or buffer items. The general rules are:

• the sets of names/expressions on both sides must be identical
• no value must appear more than once on either side, i.e. this must be prohibited: a,b,b = a,a,b
• all values on both sides of an assignment must have Python object types
• attribute access is only allowed at the C level, i.e. no Python attributes or properties
• item access (indexing) is only allowed for C types, including buffers and Python lists

[scoder]

See also ticket #426.

[scoder]

Simple local variable swapping is implemented here:

 http://hg.cython.org/cython-devel/rev/1f0d7bae2d4e

Detecting optimisable IndexNode assignments is implemented here:

 http://hg.cython.org/cython-devel/rev/43f5edccdf13

The optimisation itself is pending for index assignments, as this requires changes to __Pyx_GetItemInt() and friends.

[scoder]

Attribute access is optimised here:

 http://hg.cython.org/cython-devel/rev/93a98a1f016a

Follow-up for index access in #438.