Allow more join types, step1: comment out most of database test, leave one join test, convert existing implementation to expandable syntax
Allow more join types, step2: reintroduce simplejoin (aka old join), db::simplejoin<a,many<b>,many<c>,d> means: a has many b; each b has many c; each c has one d;
Next step, rewritten entity-to-xmlrender and it actually works now.
Remove bs tuple abstraction
Working 3 entity join