Specify (array) traversals

[judofyr]

This adds specification of how (array) traversals should work:

• A new section explaining the concept of "traversals". These are evaluated in a right-associative way.
• AttributeAccess, ElementAccess, Slice, Filter, Projection, Dereference are no longer standalone expressions, but rather part of a traversal.
• Adds all of the rules for how traversals are combined and when join/mapping/flat-mapping is happening.