Add Antithesis intrumentation (#5042)

* Copy Antithesis SDK version 0.4.0 to directory external/
* Add build option `voidstar` to enable instrumentation with Antithesis SDK
* Define instrumentation macros ASSERT and UNREACHABLE in terms of regular C assert
* Replace asserts with named ASSERT or UNREACHABLE
* Add UNREACHABLE to LogicError
* Document instrumentation macros in CONTRIBUTING.md

src/libxrpl/protocol/STLedgerEntry.cpp

@@ -161,7 +161,9 @@ STLedgerEntry::thread(
     if (oldPrevTxID == txID)
     {
         // this transaction is already threaded
-        assert(getFieldU32(sfPreviousTxnLgrSeq) == ledgerSeq);
+        ASSERT(
+            getFieldU32(sfPreviousTxnLgrSeq) == ledgerSeq,
+            "ripple::STLedgerEntry::thread : ledger sequence match");
         return false;
     }