Better modularization of StorageSlot.sol::getAddressFromSlot()

[Z323323]

getAddressFromSlot() is both used to set addresses into precalculated storage slots (returning a reference of the AddressSlot struct), and to get addresses stored into that slot. I guess it would be better to separate the setter from the getter, which I've already done in the code below. There's no need to reassign the storage slot of the struct everytime we need to read the value.