Add Array initialization from struct constructor (#5865) (#6147)

- add flatbuffers::span
- add new constructor for `struct` with `array`
- add some test for flatbuffers::span and 'arrays_test.fbs'

tests/evolution_test/evolution_v2_generated.h

@@ -131,6 +131,7 @@ FLATBUFFERS_MANUALLY_ALIGNED_STRUCT(8) Struct FLATBUFFERS_FINAL_CLASS {
       : a_(flatbuffers::EndianScalar(_a)),
         padding0__(0),
         b_(flatbuffers::EndianScalar(_b)) {
+    (void)padding0__;
   }
   int32_t a() const {
     return flatbuffers::EndianScalar(a_);