Reduced the max size for `cradle::mpl::string`.

* cradle/mpl/string.hpp:
  * Reduced the max size of `cradle::mpl::string`.
  * Added loop unrolling for recursive template instantiation.
* test/unit/mpl/string_compile.cpp: Fixed test cases.
Merge pull request #5 from Cryolite/fix/mpl_string

Reduced the max size for `cradle::mpl::string`.