[Ada] Provide extended contracts for formal doubly linked lists

[Arnaud Charlet <charlet at adacore dot com>]

Primitives of doubly linked lists are now annotated with precise
contracts which can be used for formal verification with GNATprove.

Tested on x86_64-pc-linux-gnu, committed on trunk

2017-04-27  Claire Dross  <dross@adacore.com>

	* a-cfdlli.adb, a-cfdlli.ads (=): Generic parameter removed to
	allow the use of regular equality over elements in contracts.
	(Cursor): Type is now public so that it can be used in
	model functions.
	(Formal_Model): Ghost package containing
	model functions that are used in subprogram contracts.
	(Current_To_Last): Removed, model functions should be used
	instead.
	(First_To_Previous): Removed, model functions should
	be used instead.
	(Strict_Equal): Removed, model functions
	should be used instead.
	(Append): Default parameter value
	replaced by new wrapper to allow more precise contracts.
	(Insert): Default parameter value replaced by new wrapper to
	allow more precise contracts.
	(Delete): Default parameter
	value replaced by new wrapper to allow more precise contracts.
	(Prepend): Default parameter value replaced by new wrapper to
	allow more precise contracts.
	(Delete_First): Default parameter
	value replaced by new wrapper to allow more precise contracts.
	(Delete_Last): Default parameter value replaced by new wrapper
	to allow more precise contracts.

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