Multiway join rules

Add rules to compile query to a plan using HyperCube shuffle and multiway join. 
* [x]  add rule to merge left deep join into nary join in logical plan
* [x]  add rule break nary join to hypercube shuffle and local multiway join
* [x]  add rule to instantiate local multiway join (currently we only have leapfrog, but there could be more, so this part is seperated from break break nary join)
* [x]  add rule to place sort before LeapFrogJoin.
* [x]  add unit test
* [x]  add query test
 

I notice that you only call `merge_to_multiway_join` right here. This this function is a 3-line function, why not just inline that code here? What do you gain by refactoring it out?