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.

This partially revert the change introduced in
767fce81.

Turns out AttributeNodes always start with type error, which we
don't want to propagate in some places.

Add visibility kwarg to cython.declare.

Add declarations for C++11/14 smart pointer factories (make_shared & make_unique)

DOCS: Fix typo

Don't error out when non-const value is passed as const function parameter

Silence a warning about applying unary minus to an unsigned integer

with MSVC.

function parameter.

parameter to a function. When function arguments are marked as const,
this means that the function should not modify them, not that
non-const arguments are not allowed.
This also fixes a test that began failing because this change resulted
in a slightly different error message when conversion to a const
qualified argument is not possible.

__Pyx_PyInt_TrueDivideObjC: switch comparison order

FROM_PY_FUNCTION: Cast to sdigit after unary minus, not before

The current order results in compiler warnings on 32 bit machines, e.g.

src/llfuse.c: In function '__Pyx_PyInt_TrueDivideObjC':
src/llfuse.c:43980:17: warning: left shift count >= width of type
                 if (8 * sizeof(long) <= 53 || (__Pyx_sst_abs(size) <= 52 / PyLong_SHIFT) || likely(labs(a) <= (1L << 53))) {

Switching the order so that the left shift is closer to the sizeof test
avoids the warning, presumably because it makes it easier for the
compiler to see that the left shift is only executed on 64 bit.

Thanks to Christian Neukirchen for doing most of the work!