Add std:string overload for Debug (#255)

Add an overload in debug for std:string based on the pattern for
std::string_view

sdk/include/debug.hh

diff --git a/sdk/include/debug.hh b/sdk/include/debug.hh
index 21fb6b8..af0732e 100644
--- a/sdk/include/debug.hh
+++ b/sdk/include/debug.hh
@@ -285,6 +285,19 @@
 };
 
 /**
+ * String view specialisation, use the C string handler.
+ */
+template<>
+struct DebugFormatArgumentAdaptor<std::string>
+{
+	__always_inline static DebugFormatArgument construct(std::string &value)
+	{
+		return DebugFormatArgumentAdaptor<const char *>::construct(
+		  value.c_str());
+	}
+};
+
+/**
  * Enum specialisation, prints the enum as a string and then the numeric value.
  *
  * This specialisation uses the generic printing facility in the library call

tests/test-runner.cc

diff --git a/tests/test-runner.cc b/tests/test-runner.cc
index 5ae2963..2227633 100644
--- a/tests/test-runner.cc
+++ b/tests/test-runner.cc
@@ -4,8 +4,10 @@
 #include "tests.hh"
 #include <compartment.h>
 #include <simulator.h>
+#include <string>
 
 using namespace CHERI;
+using namespace std::string_literals;
 
 namespace
 {
@@ -104,10 +106,10 @@
 	          0x123456789012345ULL);
 	const char *testString = "Hello, world! with some trailing characters";
 	// Make sure that we don't print the trailing characters
-	debug_log("Trying to print string: {}", std::string_view{testString, 13});
-	std::string hi = "Hello world";
-	debug_log("Trying to print std::string: {}", hi.c_str());
-
+	debug_log("Trying to print std::string_view: {}",
+	          std::string_view{testString, 13});
+	const std::string S = "I am a walrus"s;
+	debug_log("Trying to print std::string: {}", S);
 	run_timed("All tests", []() {
 		run_timed("Debug helpers (C++)", test_debug_cxx);
 		run_timed("Debug helpers (C)", test_debug_c);