Split MessageInfo into its own header

This is first step towards splitting apart reporter implementation
and `catch_string.hpp`.

src/catch2/internal/catch_message_info.cpp

@@ -0,0 +1,26 @@
+
+#include <catch2/internal/catch_message_info.hpp>
+
+namespace Catch {
+
+    MessageInfo::MessageInfo(   StringRef const& _macroName,
+                                SourceLineInfo const& _lineInfo,
+                                ResultWas::OfType _type )
+    :   macroName( _macroName ),
+        lineInfo( _lineInfo ),
+        type( _type ),
+        sequence( ++globalCount )
+    {}
+
+    bool MessageInfo::operator==( MessageInfo const& other ) const {
+        return sequence == other.sequence;
+    }
+
+    bool MessageInfo::operator<( MessageInfo const& other ) const {
+        return sequence < other.sequence;
+    }
+
+    // This may need protecting if threading support is added
+    unsigned int MessageInfo::globalCount = 0;
+
+} // end namespace Catch

src/catch2/internal/catch_message_info.hpp

@@ -0,0 +1,31 @@
+#ifndef CATCH_MESSAGE_INFO_HPP_INCLUDED
+#define CATCH_MESSAGE_INFO_HPP_INCLUDED
+
+#include <catch2/internal/catch_result_type.hpp>
+#include <catch2/internal/catch_common.hpp>
+#include <catch2/interfaces/catch_interfaces_capture.hpp>
+
+#include <string>
+
+namespace Catch {
+
+    struct MessageInfo {
+        MessageInfo(    StringRef const& _macroName,
+                        SourceLineInfo const& _lineInfo,
+                        ResultWas::OfType _type );
+
+        StringRef macroName;
+        std::string message;
+        SourceLineInfo lineInfo;
+        ResultWas::OfType type;
+        unsigned int sequence;
+
+        bool operator == ( MessageInfo const& other ) const;
+        bool operator < ( MessageInfo const& other ) const;
+    private:
+        static unsigned int globalCount;
+    };
+
+} // end namespace Catch
+
+#endif // CATCH_MESSAGE_INFO_HPP_INCLUDED

src/catch2/internal/catch_run_context.hpp

@@ -16,6 +16,7 @@
 #include <catch2/internal/catch_test_registry.hpp>
 #include <catch2/catch_test_case_info.hpp>
 #include <catch2/internal/catch_test_macro_impl.hpp>
+#include <catch2/catch_message.hpp>
 #include <catch2/catch_totals.hpp>
 #include <catch2/catch_test_spec.hpp>
 #include <catch2/internal/catch_test_case_tracker.hpp>