Make allocators pure.
* Methods allocating/deallocating memory are pure.
* Allocator.instance is pure (once initialized, it always returns
the same instance).
* defaultAllocator getter property is pure (should be set at the
beginning, and always return the same instance after that).
Rename and extend Allocator.expand to reallocateInPlace. The problem is
that realloc for example doesn't guarante that the shrinking of the
memory block won't cause address change. So not only expanding should
have "in place" version, but the shrinking as well.
D dereferences the pointer wrong because of missing difference between .
and -> operators, if trying to write a block over another
block. So use memmove first to move the memory and then update the fields
that should be changed (only size).
