carrot_core: refactor from tools:variant -> std::variant

src/carrot_core/lazy_amount_commitment.cpp

@@ -52,7 +52,7 @@ rct::key calculate_amount_commitment(const lazy_amount_commitment_t &lazy_amount
         rct::key operator()(const rct::xmr_amount &a) const { return rct::zeroCommit(a); }
     };
 
-    return lazy_amount_commitment.visit(lazy_amount_commitment_visitor{});
+    return std::visit(lazy_amount_commitment_visitor{}, lazy_amount_commitment);
 }
 //-------------------------------------------------------------------------------------------------------------------
 } //namespace carrot