store join parts in a tree
generate the association hash from the top down

This prevents us from walking back up the parent tree once for each
leaf.
do not convert the tree to a list just for the first node
walk the tree rather than generating a hash and walking it
rename the variable to make more sense with a tree
reduce number of comparisons and array allocations

Just inspecting the children nodes will reduce the number of comparisons
we have to do (since it is limited to the child size)
parent is guaranteed to be the same

since we are searching a tree, the current level should be consistent,
so we don't have to inspect the parent class / table
we do not need to to_s the name all the time
we will always detect the same node, so just pass the node in
eliminate function that is only used in one place
names are guaranteed to be symbols
convert JoinBase to a tree and remove the Node class
push parent up to the superclass

We always want a linked list back to the root node.
remove == so we can see where walking up parents occurs
make node search more efficient

Rather than search every node in the tree, comparing that node and all
of its parents every time, start at the root from both sides and work
our way down the tree
speed up finding existing nodes

The node must be a child of the parent passed in, so only search
children of the parent node
just skip the join if it's already there

Remove the annoying `find_or_build` stuff.
expose the root node and call it
speed up match?
hide join_constraints inside the JoinDependency object