Container data is stored in a format identical
to their C memory layout.
Instead of full deserialization, a lightweight readonly
view can be created atop of provided buffer.
This reduces amount of allocations and copying
and is suitable for creating readonly bitmaps from
memory mapped files.

* Added new functions:
  roaring_bitmap_frozen_size_in_bytes()
  roaring_bitmap_frozen_serialize()
  roaring_bitmap_frozen_view()

* copy_on_write was moved into more generic roaring_array_t.flags
  To modify copy_on_write behavior, new functions were added:
  roaring_bitmap_get_copy_on_write()
  roaring_bitmap_set_copy_on_write()

* roaring_bitmap_free() now takes pointer to a constant bitmap