Relations api and SQLGEN

I decided to split the relations api from the more advanced issues realted to the current sqlgen implementation. The main reason for this split is due to the never ending discussions about what is a relation on drupal forums, irc, mailing lists, etc...

The relations api is going to be a simple implementation of a tree and graph indexes, with emebeded SQL. You should be able to do things like - get related nodes; get next, previous, parent, children, sibling nodes.

Long term-wise we should be able to replace the book and taxonomy implementations of the database using this api. I'm intentionally going to make it extremely minimalistic, since I'm really not a supporter of using embdedded sql statements.

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