add set_tid_address
add writev
add exit_group (the same as exit)
rename wait to waitpid

fix std::map at(): return reference to T
fix std::get(tuple&&): return lvalue reference if Type is lval ref
fix rbtree: operator bool() should be non-explicit
fix rbtree: add a 4-bytes member to rbtree to keep it aligned to 16bytes
process: remove next_pid
process: changed some shitty constructors
process, syscall, vfs: change format of some container calls