Store Opt/Arg hint and description as StringRef

There is no good reason for these to be std::strings, as these
are just (optional) constants for nice user output. This ends up
reducing the allocations significantly.

When measuring allocations when running no tests, the changes are
`tests/SelfTest` 9213 -> 7705
`tests/ExtraTests/NoTests` 3723 -> 2215

src/catch2/internal/catch_clara.cpp

@@ -185,7 +185,7 @@ namespace Catch {
             }
             if (!m_hint.empty())
                 oss << " <" << m_hint << '>';
-            return { { oss.str(), m_description } };
+            return { { oss.str(), static_cast<std::string>(m_description) } };
         }
 
         bool Opt::isMatch(std::string const& optToken) const {