Fix all TODOs

[asankov]

8 ATM:

$ grep -r 'TODO' .
./chapter-4/4.3-a-library-for-others/README.md:TODO: implementations #5
./chapter-4/4.3-a-library-for-others/README.md:*Answer:* TODO: implementation #7
./chapter-3/3.6-c++/README.md:TODO: measure performance between the two versions.
./chapter-3/3.3-building-the-data-structure-in-c/structs.c:        // TODO: is that check needed, maybe we can return directly here
./chapter-3/3.4-generating-output/README.md:*TODO: add implementation*
./chapter-3/3.4-generating-output/README.md:*Answer:* *TODO: add answer and implementation*
./chapter-2/2.8-trees/README.md:TODO: execute a proper performance tests againts the list quicksort.
./chapter-2/2.7-lists/README.md:- C++ solution - TODO