fix: update balanceShares on AccountVaultPosition on transfers to/from the account

[jstashh]

fixes https://github.com/yearn/yearn-vaults-v2-subgraph/issues/54 and https://github.com/yearn/yearn-vaults-v2-subgraph/issues/53

• fix balanceShares not updating when a user transfers into/out of a vault
• create an AccountVaultPosition when a user transfers into a vault without previously depositing

I'm not sure we want to store balanceTokens on AccountVaultPosition because won't it always be out of date as the pricePerShare changes? We should just derive this property from numShares * latestVaultPosition.pricePerShare? Not sure

Also I had to do the dance around the u8 in getBalancePosition because I was getting this error when deploying the subgraph:

{
  "data": {
    "indexingStatusForCurrentVersion": {
      "fatalError": {
        "message": "out of range integral type conversion attempted\twasm backtrace:\t    0: 0x256e - <unknown>!src/utils/account/vault-position/getBalancePosition\t    1: 0x2873 - <unknown>!src/utils/account/vault-position/deposit\t    2: 0x2ac0 - <unknown>!src/utils/vault/vault/deposit\t    3: 0x2bbb - <unknown>!src/mappings/vaultMappings/handleDepositWithAmount\t in handler handleDepositWithAmount at block #11579817 (edbc0c6a8a3946bd0b01493398b73a57516e6f5258e325b86c525e942212644e)"
      },
      "nonFatalErrors": [],
      "subgraph": "QmTcCeGHq8xD4uPF2KDHZxRxWgsABgLxtGcEkapqwZFHnR"
    }
  }
}