special-case the substring list processing in JoinedStrNode again to reduce the number of temp vars used

speed up compilation for functions with large numbers of temps (already shows for a couple of thousand, e.g. for large list literals)

Docstrings don't support f-strings in cpython, so they don't need to here either. The scanner
seems to be working.

Current status:
- All but three of the tests from CPython's test_fstring pass. Two of the remaining ones rely
  on exec and the third relies on a runtime NameError that occurs at compile time in Cython. These
  tests still need to be added to the Cython test suite.
- I'm not overly convinced that the refcounting is correct in the C code that's currently
  emitted.
- There's some room for optimization, such as removing empty strings from JoinedStrNode,
  concatenating adjacent raw strings at compile time, and using _PyUnicodeWriter.
- Currently the feature is available in both Python 2 and Python 3 mode. It mostly works in
  Python 2, except for the !a conversion char.

Not sure about the refcounting

Using _PyUnicode_Writer might be faster but seems more complicated to implement.

Still doesn't do concatenation of FormattedValueNodes correctly.

Parses f-strings into ExprNodes, but with no support for compiling them. Some
known bugs are marked as TODOs.