Implemented methods on AvlTree to print F# code to recreate the tree.
Added [<StructuredFormatDisplay>] to AvlTree with a private property which calls the pretty-printing methods. Note that [<StructuredFormatDisplay>] and the CompilationRepresentation attribute we've applied can't be used together because StructuredFormatDisplay has to be used with an instance property (which doesn't work when using null for the Empty case representation).
Minor tweaks and reformatting.