1. should I rename any functions? if yes, which ones?
2. should I remove error output argument from some functions where we're most
   likely not interested in it (like `tr_sys_file_close`)? if yes, 1) what are
   those functions exactly and 2) should affected functions log errors (at
   least in debug mode)?
3. should I add any other test cases for `tr_error_*` and
   `tr_sys_(path|file|dir)_*` functions?
4. should I introduce some typedefs instead of raw uint64_t/int64_t? or should
   I even use standard size_t/off_t instead? each time I think of it I decide
   to stay with raw types, but for some reason I still think one may have
   better idea about it than me.
5.