remove format_error_for_value and add get_region instead

it is convenient to have get_region function that can access region_info
in toml::value. get_region is placed in toml::detail and made friend of
toml::value because I don't want to make toml::value::region_info public
and keep it internal use only.

toml/parser.hpp

@@ -6,6 +6,7 @@
 #include "lexer.hpp"
 #include "types.hpp"
 #include "value.hpp"
+#include <iostream>
 #include <cstring>
 
 namespace toml