Rename catch_stream -> catch_reusable_string_stream

After everything else was split out, this name much reflects the
actual contents of the file(s).

src/catch2/internal/catch_reusable_string_stream.cpp

@@ -0,0 +1,62 @@
+
+//              Copyright Catch2 Authors
+// Distributed under the Boost Software License, Version 1.0.
+//   (See accompanying file LICENSE_1_0.txt or copy at
+//        https://www.boost.org/LICENSE_1_0.txt)
+
+// SPDX-License-Identifier: BSL-1.0
+#include <catch2/internal/catch_reusable_string_stream.hpp>
+#include <catch2/internal/catch_singletons.hpp>
+#include <catch2/internal/catch_unique_ptr.hpp>
+
+#include <cstdio>
+#include <sstream>
+#include <vector>
+
+namespace Catch {
+
+    // This class encapsulates the idea of a pool of ostringstreams that can be reused.
+    struct StringStreams {
+        std::vector<Detail::unique_ptr<std::ostringstream>> m_streams;
+        std::vector<std::size_t> m_unused;
+        std::ostringstream m_referenceStream; // Used for copy state/ flags from
+
+        auto add() -> std::size_t {
+            if( m_unused.empty() ) {
+                m_streams.push_back( Detail::make_unique<std::ostringstream>() );
+                return m_streams.size()-1;
+            }
+            else {
+                auto index = m_unused.back();
+                m_unused.pop_back();
+                return index;
+            }
+        }
+
+        void release( std::size_t index ) {
+            m_streams[index]->copyfmt( m_referenceStream ); // Restore initial flags and other state
+            m_unused.push_back(index);
+        }
+    };
+
+    ReusableStringStream::ReusableStringStream()
+    :   m_index( Singleton<StringStreams>::getMutable().add() ),
+        m_oss( Singleton<StringStreams>::getMutable().m_streams[m_index].get() )
+    {}
+
+    ReusableStringStream::~ReusableStringStream() {
+        static_cast<std::ostringstream*>( m_oss )->str("");
+        m_oss->clear();
+        Singleton<StringStreams>::getMutable().release( m_index );
+    }
+
+    std::string ReusableStringStream::str() const {
+        return static_cast<std::ostringstream*>( m_oss )->str();
+    }
+
+    void ReusableStringStream::str( std::string const& str ) {
+        static_cast<std::ostringstream*>( m_oss )->str( str );
+    }
+
+
+}