cache underlying PyCFunction of calls to unbound methods of builtins in utility code (should eventually become a general feature)

Use appveyor to build windows wheels

Use the appveyor continuous integration service to build wheels for
windows. This procedure was adapted from this guide:

https://packaging.python.org/en/latest/appveyor.html

Fix the pxd file for libcpp.complex

avoid runtime 'in' tests for dict literals when we can determine at compile time that there are no duplicate keys

Fix two small sphinx errors in the tutorials

Added it to libcpp_all (where it should have been) instead
of a custom compile test.

complex isn't actually tested in libcpp_all, except that it
can be cimported

Previously it could not be cimported
Also added a test case (for cimporting the whole of the libcpp
standard library)