[Ada] Fixed bugs in iterators for vector containers

[Arnaud Charlet <charlet at adacore dot com>]

The First and Last selector functions return a value that depends on whether
this is complete or partial iteration. For complete iteration, the selector
function returns the logical beginning of the entire sequence of items in the
container. (To be specific, Container.First for a forward iterator, and
Container.Last for a reverse iterator.) For partial iteration, the selector
function returns the start position value specified when the iterator object
was constructed (in this case, both First and Last return the same value).

The Next and Previous iterator operations vet the cursor parameter to ensure
that it designates a node in the same container as the iterator. The function
then forwards to the call to the analogous cursor-based operation.

Iterate constructs an iterator object whose state indicates whether this is
complete or partial iteration. There was also change in the semantics of the
partial iterator (per the ARG meeting in Denver on 2011/11): if the start
position equals No_Element, then it raises Constraint_Error; otherwise, it
constructs an iterator object to indicate the position from which the iteration
begins (which is in turn used by the selector functions First and Last).

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

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

	* a-convec.adb, a-coinve.adb, a-cobove.adb (Iterator): Use
	subtype Index_Type'Base for Index component (Finalize): Remove
	unnecessary access check (First, Last): Cursor return value
	depends on iterator index value (Iterate): Use start position as
	iterator index value (Next, Previous): Forward to corresponding
	cursor-based operation.
	* a-cborma.adb (Iterate): Properly initialize iterator object (with 0
	as node index).

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