[Ada] Add contracts to Ada.Strings.Fixed

[Pierre-Marie de Rodat]

This adds complete postconditions and contract cases to subprograms in
Ada.Strings.Fixed.

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

gcc/ada/

	* libgnat/a-strfix.ads: Add postconditions and contract cases to
	subprograms.
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