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

include/xrpl/basics/Slice.h

@@ -22,9 +22,9 @@
 
 #include <xrpl/basics/contract.h>
 #include <xrpl/basics/strHex.h>
+#include <xrpl/beast/utility/instrumentation.h>
 #include <algorithm>
 #include <array>
-#include <cassert>
 #include <cstdint>
 #include <cstring>
 #include <limits>
@@ -103,7 +103,9 @@ public:
     std::uint8_t
     operator[](std::size_t i) const noexcept
     {
-        assert(i < size_);
+        ASSERT(
+            i < size_,
+            "ripple::Slice::operator[](std::size_t) const : valid input");
         return data_[i];
     }