Update account terminology in contracts

[0x-r4bbit]

Throughout the development of this protocol we've used User and Account whenever we referred to a vault.
Now that we're introducing the notion of "owners" of vaults with #71 we should rethink the wording here.

I'd propose that:

• The StakeManager only knows about vaults. We know that the contracts interacting with the protocol have to be whitelisted vaults, so there's no need to think of "smart accounts".
• Whenever we refer to "owners" of vaults, we should probably just name those accounts (this covers both, EOAs and smart accounts)

As a result of that, we should probably do the following changes:

staking-reward-streamer/src/RewardsStreamerMP.sol

Line 58 in c8ea850

struct Account {

should be VaultData

staking-reward-streamer/src/RewardsStreamerMP.sol

Line 67 in c8ea850

mapping(address vault => Account data) public accounts;

should be vaults

staking-reward-streamer/src/RewardsStreamerMP.sol

Line 117 in c8ea850

function getUserTotalMP(address user) external view returns (uint256) {

should be getAccountTotalMP

staking-reward-streamer/src/RewardsStreamerMP.sol

Line 134 in c8ea850

function getUserTotalMaxMP(address user) external view returns (uint256) {

should be getAccountTotalMaxMP

staking-reward-streamer/src/RewardsStreamerMP.sol

Line 150 in c8ea850

function getUserTotalStakedBalance(address user) external view returns (uint256) {

should be getAccountTotalStakedBalance

staking-reward-streamer/src/RewardsStreamerMP.sol

Line 445 in c8ea850

function updateAccountMP(address accountAddress) external onlyNotEmergencyMode {

should be updateVaultMP

staking-reward-streamer/src/RewardsStreamerMP.sol

Line 469 in c8ea850

function getAccount(address accountAddress) external view returns (Account memory) {

should be getVaultData

and so on and so forth