Debugging quality-of-life improvements (#810)

* Debugging quality-of-life improvements

- Use std::abort for `CHECK`/`FATAL` failures, which acts as a debugger breakpoint as well as automatically printing a stack trace.
- Re-enable printing continuations in `--trace` mode.
- Log the source location of each step in `--trace` mode.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/interpreter/value.cpp

@@ -9,6 +9,7 @@
 #include "common/check.h"
 #include "executable_semantics/common/arena.h"
 #include "executable_semantics/common/error.h"
+#include "executable_semantics/interpreter/frame.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Casting.h"
 
@@ -232,11 +233,15 @@ void Value::Print(llvm::raw_ostream& out) const {
     case Value::Kind::VariableType:
       out << cast<VariableType>(*this).Name();
       break;
-    case Value::Kind::ContinuationValue:
-      out << "continuation";
-      // TODO: Find a way to print useful information about the continuation
-      // without creating a dependency cycle.
+    case Value::Kind::ContinuationValue: {
+      out << "{";
+      llvm::ListSeparator sep(" :: ");
+      for (Ptr<Frame> frame : cast<ContinuationValue>(*this).Stack()) {
+        out << sep << *frame;
+      }
+      out << "}";
       break;
+    }
     case Value::Kind::StringType:
       out << "String";
       break;