Use single balances in the Stellar Asset Contract (#615)

* Use single balances for classic accounts in the classic asset scenario

* Add method for spendable balance

* add more tests

* Fix error when an unexpected LedgerEntry type is returned

* allow burn even if balance is frozen/deauthorized

* Added TODO's for metering analysis

* remove pub

* Return clear error when account or trustline is missing

* clear up overflow error

* Use helpers to unify asset4 and asset12 cases

* sp_balance to spendable

* Specify all Identifier types on match