[Ada] Use Stream_Element_Arrays internally for secure hash computations

[Ada] Use Stream_Element_Arrays internally for secure hash computations

gcc/ada/

	* libgnat/g-sechas.ads, libgnat/g-sechas.adb: Refactor to use
	Stream_Element_Array as the internal buffer type.
	* libgnat/g-shshco.adb: Adjust to use Stream_Element_Offset
	instead of Integer as the index in the internal state buffer.