Save gas by caching array length used in for loops

[code423n4]

Handle

0x0x0x

Vulnerability details

Proof of Concept

Example:

for (uint i = 0; i < arr.length; i++) {
//Operations not effecting the length of the array.
}

Loading length of array costs gas. Therefore, the length should be cached, if the length of the array doesn't change inside the loop.
Recommended implementation:

uint length = arr.length;
for (uint i = 0; i < length; i++) {
//Operations not effecting the length of the array.
}

By doing so the length is only loaded once rather than loading it as many times as iterations (Therefore, less gas is spent).

Occurences

contracts/FeeSplitter.sol:108:        for (uint256 i = 0; i < _accounts.length; i++) {
contracts/FeeSplitter.sol:125:        for (uint256 i = 0; i < _tokens.length; i++) {
contracts/FeeSplitter.sol:210:        for (uint256 i = 0; i < shareholders.length; i++) {
contracts/FeeSplitter.sol:227:        for (uint256 i = 0; i < shareholders.length; i++) {
contracts/MixinOperatorResolver.sol:32:        for (uint256 i = 0; i < requiredAddresses.length; i++) {
contracts/MixinOperatorResolver.sol:48:        for (uint256 i = 0; i < requiredAddresses.length; i++) {
contracts/NestedFactory.sol:203:        for (uint256 i = 0; i < tokens.length; i++) {
contracts/NestedFactory.sol:280:        for (uint256 i = 0; i < _orders.length; i++) {
contracts/NestedFactory.sol:316:        for (uint256 i = 0; i < _orders.length; i++) {
contracts/OperatorResolver.sol:33:        for (uint256 i = 0; i < names.length; i++) {
contracts/OperatorResolver.sol:45:        for (uint256 i = 0; i < names.length; i++) {
contracts/OperatorResolver.sol:56:        for (uint256 i = 0; i < destinations.length; i++) {

[maximebrugel]

Some examples :

With FeeSplitter::releaseTokens (doesn't optimize with one iteration)
Before :

After :

With NestedFactory::destroy (doesn't optimize with one iteration)
Before:

After:

With feeSplitter::setShareholders (optimize with two interations)
Before:

After:

[maximebrugel]

In most cases, this will not change anything (or even make it worse) if there is mostly one or two iterations.

We will implement this if there are frequently several iterations.