Don't verify transactions twice

[ljedrz]

When running Ledger::add_next_block, the transactions are currently verified twice:

• in Ledger::check_next_block (called at vm/compiler/src/ledger/mod.rs:508)
• in VM::finalize (called at vm/compiler/src/ledger/mod.rs:556)

Since this isn't a fast operation, we should only be doing it once, likely just in Ledger::check_next_block.

[ljedrz]

If extra safety is desired, we should instead use types, e.g. introduce a VerifiedBlock and/or a VerifiedTransaction that have the same contents as their unverified counterparts and are returned by e.g. Ledger::check_next_block (or, alternatively introduce some Verified<T> wrapper struct). They would later used by methods that require verified input, and thus don't need to verify it on their own anymore.

[ljedrz]

Cc https://github.com/AleoHQ/snarkVM/issues/986

[howardwu]

This has been fixed in testnet3.2.