Clara::Opt::getHelpColumns returns single item

It could never return multiple items, but for some reason it was
wrapping that single item in a vector.

This change ends up removing a significant (~150) number of
allocations when Catch2 has to collate the output for `-h`.

src/catch2/internal/catch_clara.cpp

@@ -173,7 +173,7 @@ namespace Catch {
         Opt::Opt(bool& ref) :
             ParserRefImpl(std::make_shared<Detail::BoundFlagRef>(ref)) {}
 
-        std::vector<Detail::HelpColumns> Opt::getHelpColumns() const {
+        Detail::HelpColumns Opt::getHelpColumns() const {
             std::ostringstream oss;
             bool first = true;
             for (auto const& opt : m_optNames) {
@@ -185,7 +185,7 @@ namespace Catch {
             }
             if (!m_hint.empty())
                 oss << " <" << m_hint << '>';
-            return { { oss.str(), static_cast<std::string>(m_description) } };
+            return { oss.str(), static_cast<std::string>(m_description) };
         }
 
         bool Opt::isMatch(std::string const& optToken) const {
@@ -311,8 +311,7 @@ namespace Catch {
         std::vector<Detail::HelpColumns> Parser::getHelpColumns() const {
             std::vector<Detail::HelpColumns> cols;
             for ( auto const& o : m_options ) {
-                auto childCols = o.getHelpColumns();
-                cols.insert( cols.end(), childCols.begin(), childCols.end() );
+                cols.push_back(o.getHelpColumns());
             }
             return cols;
         }

src/catch2/internal/catch_clara.hpp

@@ -571,7 +571,7 @@ namespace Catch {
                 return *this;
             }
 
-            std::vector<Detail::HelpColumns> getHelpColumns() const;
+            Detail::HelpColumns getHelpColumns() const;
 
             bool isMatch(std::string const& optToken) const;