Add StringCRef. (#75)
Add tests for StringCRef. (#75)
Use StringCRef instead of String. (#75)
Remove a duplicate alias definition for Text.
Add a test for StringCRef. (#75)
Add new String. (#75)
Add tests for new String. (#75)
Add assign() and append() for convenience. (#75)
Add a test for String::append(). (#75)
Fix a bug that String::append() fails to append itself. (#75)
Add a test for String. (#75)
Update ConstantNode to use String instead of std::string. (#75)
Update TreeIndex to use String instead of std::string. (#75)
Update Name to use String. (#75)