Add helloworld.pyx demo

src/Makefile

+%.so: %.pyx
+	python3 setup.py build_ext --inplace $* && mv $*.*.so $@
+
+%: %.so
+	@echo ">>> import $@" && python3 -c "import $@"
+
+clean:
+	rm -rf build
+	rm -rf *.cpp *.c
+	rm -rf *.so
+
+.PHONY: clean
+.PRECIOUS: %.so
+.SUFFIXES:

src/helloworld.pyx

+from stdlib.string cimport Str
+from libc.stdio cimport puts
+
+def main():
+    with nogil:
+        hello = Str("hello")
+        world = Str("world")
+        hello_world = hello + world
+        puts(Str.to_c_str(hello_world))
+
+        hello_again = hello_world.substr(0, 5)
+        puts(Str.to_c_str(hello_again))
+
+        world_again = hello_world.substr(5, 10)
+        puts(Str.to_c_str(world_again))
+
+main()

src/setup.py

+from distutils.core import setup
+from distutils.extension import Extension
+from Cython.Build import cythonize
+
+import sys
+src = sys.argv[-1]
+sys.argv[:] = sys.argv[:-1]
+
+extensions = [
+    Extension(
+        src,
+        language='c++',
+        sources=[src + '.pyx'],
+        extra_compile_args=['-std=c++17'],
+    )
+]
+
+setup(
+    ext_modules = cythonize(extensions)
+)
+