add +gobra headers in all packages (#22)

Adds a //+gobra comment to every package to ensure usability with the
`--onlyFilesWithHeader` flag

dicts/dicts.gobra

@@ -13,6 +13,7 @@
     - https://www.why3.org/stdlib/fmap.html
     - https://www.why3.org/stdlib/map.html
 */
+//+gobra
 
 // This package defines lemmas for mathematical maps (aka dictionaries) commonly used
 // in specifications.

gomap/gomap.gobra

@@ -3,6 +3,7 @@
   See LICENSE or go to https://github.com/viperproject/gobra-libs/blob/main/LICENSE
   for full license details.
 */
+//+gobra
 
 // This package defines Go maps and their associated operations in terms of
 // mathematical maps (dictionaries).

math/math.gobra

@@ -1,3 +1,4 @@
+//+gobra
 // This package defines mathematical operations commonly used in specifications.
 package math
 

seqs/seqs.gobra

@@ -6,6 +6,7 @@
   This file is inspired by the standard libraries and axiomatisations of the following verifiers:
   - dafny-lang/libraries: https://github.com/dafny-lang/libraries/blob/master/src/Collections/Sequences/Seq.dfy
 */
+//+gobra
 
 // This package defines lemmas for sequences commonly used in specifications.
 package seqs

sets/sets.gobra

@@ -10,6 +10,7 @@
   - viperproject/silicon: https://github.com/viperproject/silicon/blob/master/src/main/resources/dafny_axioms/sets.vpr
 
 */
+//+gobra
 
 // This package defines lemmas for sets commonly used in specifications.
 package sets

util/util.gobra

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//+gobra
 package util
 
 type Unit struct{}
@@ -50,4 +51,4 @@ requires b
 decreases
 pure func Asserting(ghost b bool) Unit {
 	return Unit{}
-}
+}