A better index type lookup

I've been recently distracted with other things, and also I've been working on the joins, before writing about them. The joins were actually some of the first templates I wrote, but that's also why they look a bit dated compared to the newer stuff. I probably won't do all I wanted to do with them, but I'm at least cleaning them up a bit and adding some of the more recent features. This rework takes time.

Along that way, I've extracted the logic that looks up the index types by path from SimpleAggregator and put it for reuse into TableType.  Now it can get used like this:

$indexType = $tableType->findIndexPath("primary", "fifo");

The arguments form a path of names in the index type tree. If the path is not found, the function would confess (that is die, with a stack trace). An empty path is also illegal and would cause the same result.