[Ada] Implement iterator for multiset containers

[Arnaud Charlet <charlet at adacore dot com>]

This change implements a reversible iterator for multiset containers.

An iterator can either be partial, visiting only some of the items in the
container (in which case the start position is specified), or complete,
visiting all of them.  The iterator caches the start position during its
construction, and that position value is used by First and Last to determine
their associated return values.

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

2011-12-02  Matthew Heaney  <heaney@adacore.com>

	* a-coormu.ads, a-ciormu.ads: Declare iterator factory function.
	* a-ciormu.adb, a-ciormu.adb (Iterator): Declare concrete
	Iterator type.
	(Finalize): Decrement busy counter.
	(First, Last): Cursor return value depends on iterator node value.
	(Iterate): Use start position as iterator node value.
	(Next, Previous): Forward to corresponding cursor-based operation.

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