Add full index cursoring, refactor searching

Add full forward and backward index cursoring ability to Innodb::Index.
This required first cleaning up record cursoring in Innodb::Page::Index
as well as writing backward cursoring there.

Additionally, some more refactoring was required:
  * Move the Innodb::Index @debug instance variable to be a global state
    in Innodb class. (Innodb.debug?)
  * Move the Innodb::Index @stats instance variable to be a global stats
    counter framework in Innodb::Stats. (Innodb::Stats.increment :foo)
  * Move the linear and binary search methods which only dealt with a
    single page to Innodb::Page::Index instead of Innodb::Index.
  * Move record comparison to Innodb::Record instead of Innodb::Index as
    it is now needed in both Innodb::Index and Innodb::Page::Index.