Add format.pyx demo

src/format.pyx

+from stdlib.string cimport Str
+from stdlib.format cimport format
+from libc.stdio cimport puts
+
+def main():
+    with nogil:
+        s = format("{} is {}{}", Str('forty two') , b'4', 2)
+        puts(Str.to_c_str(s))
+
+main()
+