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