Collect examples for chapter 3

chapter_03/examples/CMakeLists.txt

chapter_03/examples/fold_xpr_demo2.cc

chapter_03/examples/fold_xpr_demo3.cc

chapter_03/examples/fold_xpr_demo4.cc

chapter_03/examples/foldex.cc

chapter_03/examples/foldex_3.cc

chapter_03/examples/gcd_w_concepts.cc

chapter_03/examples/generic_func1.cc

chapter_03/examples/generic_func2.cc

chapter_03/examples/generic_func3.cc

chapter_03/examples/generic_func4.cc

chapter_03/examples/no_textsub.cc

chapter_03/examples/overload_w_concepts.cc

chapter_03/examples/print_tuple.cc

chapter_03/examples/print_tuple_cxx17.cc

chapter_03/examples/print_tuple_foldex.cc

chapter_03/examples/static_assert0.cc

chapter_03/examples/static_assert1.cc

chapter_03/examples/static_assert2.cc

chapter_03/examples/template_intro.cc