Address Robert's feedback

*** 43,53 ****
  size_t str_length(const str_t *);
  boolean_t str_copy(const str_t *, str_t *);
  void str_set(str_t *, const char *, size_t);
  boolean_t str_append(str_t *, const char *, size_t);
  boolean_t str_append_str(str_t *, const str_t *);
! boolean_t str_append_c(str_t *, int);
  boolean_t str_insert(str_t *, size_t, const char *, size_t);
  boolean_t str_insert_str(str_t *, size_t, const str_t *);
  boolean_t str_erase(str_t *, size_t, size_t);
  
  str_pair_t *str_pair_init(str_pair_t *, sysdem_ops_t *);
--- 43,53 ----
  size_t str_length(const str_t *);
  boolean_t str_copy(const str_t *, str_t *);
  void str_set(str_t *, const char *, size_t);
  boolean_t str_append(str_t *, const char *, size_t);
  boolean_t str_append_str(str_t *, const str_t *);
! boolean_t str_append_c(str_t *, char);
  boolean_t str_insert(str_t *, size_t, const char *, size_t);
  boolean_t str_insert_str(str_t *, size_t, const str_t *);
  boolean_t str_erase(str_t *, size_t, size_t);
  
  str_pair_t *str_pair_init(str_pair_t *, sysdem_ops_t *);