Added `cradle::polymorphic_allocator`.

* jamroot: Added copyright. Added an alias to `src/memory`.
* cradle/memory/polymorphic_allocator.hpp: New. Added interfaces of
  `cradle::polymorphic_allocator`.
* src/memory/jamfile: New.
* src/memory/polymorphic_allocator.cpp: New. Added implementation of
  `cradle::polymorphic_allocator`.
* test/unit/memory/jamfile: New.
* test/unit/memory/polymorphic_allocator_run.cpp: New. Added unit tests of
  `cradle::polymorphic_allocator`.