[Ada] Proof of Ada.Strings.Maps
gcc/ada/
* libgnat/a-strmap.adb: Add ghost code for proof.
(To_Range): This is the most involved proof, as it requires
creating the result of the call to To_Domain as a ghost
variable, and show the unicity of this result in order to prove
the postcondition.
* libgnat/a-strmap.ads: (SPARK_Proof_Sorted_Character_Sequence):
New ghost function.
(To_Domain): Add postcondition regarding sorting of result.
(To_Range): Fix postcondition that should compare Length instead
of Last for the results of To_Domain and To_Range, as the value
of Last for an empty result is not specified in the Ada RM.