Alpha/Michelson/Migration: double hard_gas_limit_per_{operation,block}

This patch is part of the migration code from `alpha_003` to `athens_004`.

It performs the migration of the constants in mainnet to update them
to the new (doubled) default values.

src/proto_alpha/lib_protocol/src/init_storage.ml

@@ -45,6 +45,12 @@ let prepare_first_block ctxt ~typecheck ~level ~timestamp ~fitness =
       Vote_storage.freeze_listings ctxt >>=? fun ctxt ->
       return ctxt
   | Alpha_previous ->
+      (* Update hard gas limits to the new defaults. *)
+      Raw_context.patch_constants ctxt begin fun constants ->
+        { constants with
+          hard_gas_limit_per_operation = Constants_repr.default.hard_gas_limit_per_operation ;
+          hard_gas_limit_per_block = Constants_repr.default.hard_gas_limit_per_block }
+      end >>= fun ctxt ->
       return ctxt
 
 let prepare ctxt ~level ~timestamp ~fitness =