Add fuzzer only invariants

[MonsieurNicolas]

When fuzzing the transaction code, we detect:

• crashes
• aborts from the c++ runtime (extra checks)
• protocol invariant failures
• "internal errors" detected by the transaction subsystem

When fuzzing, we can add invariants (in a general sense, not necessarily "protocol invariants" that must obey certain rules) that take advantage of the fact that the ledger size is very small, so we can perform checks on the entire ledger and perform all those checks in memory.

Over time, we may find that subsets of those new invariants can be integrated into regular protocol invariants.

This will increase the number of "bad things" detected while fuzzing.

[MonsieurNicolas]

List of things we should be looking into:

• order book state (like crossed order book)
• integrity checks spanning ledger entries - I don't remember if sponsorship and ledger subentries can be fully validated with our existing invariants

[jonjove]

Resolved by #2944, #2959. Checks for sponsorship and subentries already existed.