Use initialize_copy instead of Marshal to get a deep clone of the Arel AST

[mvgijssel]

Currently

Marshal.load(Marshal.dump(object))

is used to get a deep clone of an Arel AST which is necessary when modifying the tree. Using Marshal feel like a hack to get a deep clone, Ruby's initialize_copy feels like a better solution to this problem.

To achieve this all Arel nodes need to implement initialize_copy when their instance variables contain a complex object, a non scalar like String or Integer.