Rename Char to xchar

To follow the naming convention of STL, the type name is no longer
capitalized. The "x" prefix, which will also be used for other
character-related types like "xstring", was carefully chosen to avoid
conflict with existing types like "wstring".
Rename String to xstring
Adopt STL naming convention in CommonResult.hh
Add *_test to .gitignore

*Test will be removed in the future in favor of *_test.
Adopt STL naming convention in ContainerHelper.hh
Adopt STL naming convention in Copy.hh
Remove DIRECT_INITIALIZE

It doesn't look very useful...
Rename DirectInitialize to direct_initialize