Rework key prefix matching

Use C string comparison instead of C++ function std::mismatch to increase
performance.

libgnucash/engine/kvp-frame.hpp

@@ -242,11 +242,8 @@ void KvpFrame::for_each_slot_prefix(std::string const & prefix,
     std::for_each (m_valuemap.begin(), m_valuemap.end(),
         [&prefix,&func](const KvpFrameImpl::map_type::value_type & a)
         {
-            std::string temp_key {a.first};
-            if (temp_key.size() < prefix.size())
-                return;
             /* Testing for prefix matching */
-            if (std::mismatch(prefix.begin(), prefix.end(), temp_key.begin()).first == prefix.end())
+            if (strncmp(a.first, prefix.c_str(), prefix.size()) == 0)
                 func (&a.first[prefix.size()], a.second);
         }
     );
@@ -259,11 +256,8 @@ void KvpFrame::for_each_slot_prefix(std::string const & prefix,
     std::for_each (m_valuemap.begin(), m_valuemap.end(),
         [&prefix,&func,&data](const KvpFrameImpl::map_type::value_type & a)
         {
-            std::string temp_key {a.first};
-            if (temp_key.size() < prefix.size())
-                return;
             /* Testing for prefix matching */
-            if (std::mismatch(prefix.begin(), prefix.end(), temp_key.begin()).first == prefix.end())
+            if (strncmp(a.first, prefix.c_str(), prefix.size()) == 0)
                 func (&a.first[prefix.size()], a.second, data);
         }
     );