Store currency/security values/prices using big.Rat natively

This adds 'shadow' types used only by the store/db internal package whch
handle converting these types to their DB-equivalent values. This change
should allow reports to be generated significantly faster since it
allows a large portion of the computation to be shifted to the database
engines.

internal/handlers/transactions.go

@@ -31,9 +31,8 @@ func GetTransactionImbalances(tx store.Tx, t *models.Transaction) (map[int64]big
 			}
 			securityid = account.SecurityId
 		}
-		amount, _ := t.Splits[i].GetAmount()
 		sum := sums[securityid]
-		(&sum).Add(&sum, amount)
+		(&sum).Add(&sum, &t.Splits[i].Amount.Rat)
 		sums[securityid] = sum
 	}
 	return sums, nil