separate transaction/certificate self-validation/ledger-validation

[vincenthz]

Separate the ledger checks for transaction (w or w/ certificate) that have to do with self-validation (making sure the format is ok, and that basic property is enforced correctly), and the context based validation.

e.g.:

X.valid_self() -> Result<(), Error>
X.apply_ledger(ledger: &Ledger) -> Result<Ledger, Error>

this is useful when wanting to make simple verification on transaction without having the Ledger available.