Put utils::tolower definition in its own cpp file

src/utils/tolower.cpp

@@ -0,0 +1,13 @@
+#include <utils/tolower.hpp>
+
+namespace utils
+{
+  std::string tolower(const std::string& original)
+  {
+    std::string res;
+    res.reserve(original.size());
+    for (const char c: original)
+      res += static_cast<char>(std::tolower(c));
+    return res;
+  }
+}

src/utils/tolower.hpp

@@ -5,14 +5,7 @@
 
 namespace utils
 {
-  std::string tolower(const std::string& original)
-  {
-    std::string res;
-    res.reserve(original.size());
-    for (const char c: original)
-      res += static_cast<char>(std::tolower(c));
-    return res;
-  }
+  std::string tolower(const std::string& original);
 }
 
 #endif // SPLIT_INCLUDED