* More comments.
* Refactor.
! Use <=> operator when sorting.

Used ==, < and > before, but they aren't implemented on all objects. However,
<=> must be implemented on all comparable objects.
+ #flatten! works.
* More comments.
- Make #sort! not implemented.
* Make old_insert private.
+ SortedArrayBinary.new accepts sort block.

I.e. SortedArrayBinary.new { |a, b| b <=> a }.