Gas Optimizations

[code423n4]

x = x + y is a cheaper operation than x += y

There are multiple instances in the code VaultTracker.sol

vlt.redeemable += interest;
vlt.notional += a;

vlt.redeemable += interest;

from.redeemable += interest;

to.redeemable += newVaultInterest;
to.notional += a;

sVault.redeemable += interest;

sVault.notional += a;

Swivel.sol

filled[hash] += a;

for loops length comparison can use != len instead of < len for cheaper gas

#L85 #L247 #L409 #L552

use constant/literals (e.g. type(uint256).max) instead of 2**x

#L549

++i will consume less gas than i++ if the optimizer isn't enabled

#L100 #L269 #L418 #L564

[robrobbins]

some changes done elsewhere

!= vs < implemented : https://github.com/Swivel-Finance/gost/pull/430