restructure fragment application to ledger

[vincenthz]

need to restructure the ledger checks / applications to separate some parts, which would give the ability to do more check on fragments ahead of application.

Errors need to splitted in two categories:

• ErrorStateless: the fragment is not valid and never will be : e.g. structure is invalid, some cryptographic signature return Failed, ...
• ErrorLedgerState: the fragment is not currently valid : it doesn't apply to the current ledger state (e.g. account doesn't have enough value), but could validly apply to a different ledger state.

Operations need to separate the stateless checks on a fragment from the stateful checks

Stateless:

• Structural/Consistency checks
• Limits
• Hash calculation
• Cryptographic signatures 1/2 (public key available)

Stateful:

• Cryptographic signatures 2/2 (public key need to be retrieved from state)
• Application to the state (e.g. spending money, creating a new entry)

strongly related to #7