runtests: include the C compiler error output in the compile exception to show it at the end of the test run.

Turns out that typeid() and C++ operators already work with "cpp_locals".

Add a warning in the docs that "conditional compilation" is not a good way to do most things that users commonly do with it.

See the discussion in https://github.com/cython/cython/issues/4287

Fix compilation of "ParseTreeTransforms" in Py2. It previously tried to compare the 'directivetype' to 'str', which misses Py2 str when compiling with language level 3.

Add CI jobs that compile more Cython modules with "--cython-compile-all", to make sure that this keeps working and to allow comparing the performance difference.

Fixes https://github.com/cython/cython/issues/3174
Closes https://github.com/cython/cython/pull/3175

I've removed all identification of is_self_arg from the parser, since I think it's better dealt with when analysing the declarations.

Original test copied from https://github.com/cython/cython/pull/3175

Fixes https://github.com/cython/cython/issues/4308

Make a helper function in a C++ test correctly propagate exceptions so that it won't have to spit out compiler warnings.

See https://github.com/cython/cython/issues/4187

Rewrite the section on fused types in function signatures to make it clearer how multiple fused types interact here.

Refactor function to make it less verbose and simpler to change: error cases first, then special cases, then normal behaviour.

Make code more readable by changing the order of a condition and moving the simple return case first.

For example, if they're called on entry/exit to a decorated function they pick up the directives.
They should really be independent of most user defined settings, especially local ones.

Add --skip-interpreter option to cygdb

* Fix class attributes access where cpp_locals=False.
* Add "no-cpp-locals" tag to mark a test as not suitable for running with "cpp_locals=True".
* Add a list of "extra_directives" to runtests as additional test mode.

Resolves some issues in https://github.com/cython/cython/issues/4266

Seems a remanant of an earlier implementation, but generator expressions *never* use "GeneratorExpressionScope" and in fact it's only used for comprehensions.