fix scoping rules for comprehensions and inlined generator expressions by injecting a separate scope instance

I noticed two failures to compile that I think are bugs:
 - "ctypedef Foo[int] Bar" gives a syntax error (i.e., there doesn't seem to be any way to typedef a templated type).
 - Defining a struct in a namespace and trying to convert it to a Python dict doesn't work. Instead you get nasty "__pyx_convert__to_py_THENAMESPACE" errors from the C++ compiler (it doesn't mangle the cname properly and tries to define __pyx_convert__to_py_THENAMESPACE::THESTRUCT).

I made a patch to *add tests* for the two bugs (not fix them).

prevent redundant coercion from Py_UNICODE to a unicode string when the subscript index is a Python object - this case is no longer optimised

fix test py_unicode_type, remove code from Optimize.py that became redundant when Py_UNICODE started coercing to unicode by default