Add Pluggable Index Compare Functions

Chatting with Robert Collins and Clint Bynum on IRC about postgres user defined btree functions, I realized that we could actually do the same thing pretty easily (except in the drizzle way, via plugins) We already pass callback cmp functions to the storage engines to handle collations. So all we need to do is hook in there and allow index hint to override that it's not getting the baked in callback but the plugin one instead.

In fact, this could quite simply and easily lead to pluggable collations. (collation should honestly be on index and not on column anyway) But I get ahead of myself.