add comments to each lib function referring to corresponding client and vice versa

[nlsandler]

E.g. in tests/chapter11/valid/long_expressions/long_args_client.c, add a comment:

// defined in tests/chapter11/valid/long_expressions/long_args.c
int test_sum(int a, int b, int c, long d, int e, long f, int g, int h, long i); 

and in tests/chapter11/valid/long_expressions/long_args.c, add a comment:

// called from tests/chapter11/valid/long_expressions/long_args_client.c

Suggested in #5 (review)