`attempt to add with overflow` happened in a % b

[mohanson]

New bugs happened.

use numext_fixed_uint::U4096;

fn main() {
    let a = U4096::from_dec_str("77207453359070457604620928442586248569527320043595348948921733518358535471105").unwrap();
    let b = U4096::from_dec_str("115792089237316195423570985008687907853269984665640564039457584007913129639936").unwrap();

    let r1 = &a * &a;
    println!("{}", r1);

    let r2 = r1 % b;
    println!("{}", r2);
}

thread 'test_state_work' panicked at 'attempt to add with overflow'

Originally posted by @mohanson in #35 (comment)

[mohanson]

Both happened in "v0.1.2" and "fix-div-slow-bug" branch by @yangby-cryptape

[chaoticlonghair]

rust-numext/constructor/src/fixed_uint/core/internal/private_ops.rs

Lines 536 to 539 in 1481b3d

	if of {
	// `ret[ret_idx+1]+1` could not overflow
	ret[ret_idx + 1] += 1;
	}

This impossible overflow was break by an old commit (that commit was squashed, so I cannot paste the link, that commit could estimate larger, so more code was required, if always doing estimate smaller, it can skip a lot of overflow).