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/basics/contract.cpp

@@ -19,6 +19,7 @@
 
 #include <xrpl/basics/Log.h>
 #include <xrpl/basics/contract.h>
+#include <xrpl/beast/utility/instrumentation.h>
 #include <cstdlib>
 #include <iostream>
 
@@ -48,6 +49,12 @@ LogicError(std::string const& s) noexcept
 {
     JLOG(debugLog().fatal()) << s;
     std::cerr << "Logic error: " << s << std::endl;
+    // Use a non-standard contract naming here (without namespace) because
+    // it's the only location where various unrelated execution paths may
+    // register an error; this is also why the "message" parameter is passed
+    // here.
+    // For the above reasons, we want this contract to stand out.
+    UNREACHABLE("LogicError", {{"message", s}});
     detail::accessViolation();
 }