Refactoring files for `cradle::polymorphic_allocator`.

* jamroot: Tweaked quotations.
* cradle/memory/memory_resource.hpp: Moved the definition of
  `cradle::memory_resource` from `cradle/memory/polymorphic_allocator.hpp`.
* cradle/memory/memory_resource_adaptor.hpp: Moved the definition of
  `cradle::memory_resource_adaptor` from
  `cradle/memory/polymorphic_allocator.hpp`.
* cradle/memory/polymorphic_allocator.hpp: See above.
* src/memory/jamfile: Added dependency.
* src/memory/polymorphic_allocator.cpp -> src/memory/memory_resource.cpp:
  Renamed.
* test/unit/memory/jamfile: Removed unnecessary dependencies.