custr_rremove
int custr_rremove(custr_t *, size_t, size_t);
FAIL(custr_rremove(cu, 6, 2), EINVAL);
FAIL(custr_rremove(cu, 2, 10), EINVAL);
VERIFY0(custr_rremove(cu, 1, 2));
VERIFY0(custr_rremove(cu, 1, 2));