use emplace

Allows removing the constructor name as emplace forwards the arguments
directly.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

test/lexer.t.cpp

@@ -105,7 +105,7 @@ int main (int, char**)
   while (l2.token (token, type))
   {
     std::cout << "# «" << token << "» " << Lexer::typeName (type) << "\n";
-    tokens.push_back (std::pair <std::string, Lexer::Type> (token, type));
+    tokens.emplace_back (token, type);
   }
 
   t.is (tokens[0].first,                     "one",           "tokens[0] = 'one'"); // 30
@@ -147,7 +147,7 @@ int main (int, char**)
   while (l3.token (token, type))
   {
     std::cout << "# «" << token << "» " << Lexer::typeName (type) << "\n";
-    tokens.push_back (std::pair <std::string, Lexer::Type> (token, type));
+    tokens.emplace_back (token, type);
   }
 
   t.is ((int)tokens.size (),     7,                           "7 tokens");