Fix misalignment of small structs in a Vector (C++) (#7883)

* Rare fix: kick fix test

* Rare fix: real fix

* Rare fix: separate test

* Rare fix: remove comments

* Rare fix: updates

* Rare fix: less

* Rare fix: size_t switch to uoffset_t

* Rare fix: swap exp/val

* Rare fix: add annotated before/after

* Rare fix: remove unnecessary changes

---------

Co-authored-by: Derek Bailey <derekbailey@google.com>

tests/alignment_test.fbs

@@ -21,4 +21,15 @@ table BadAlignmentRoot {
   small: [BadAlignmentSmall];
 }
 
-root_type BadAlignmentRoot;
+// sizeof(JustSmallStruct) == 2
+// alignof(JustSmallStruct) == 1
+struct JustSmallStruct {
+  var_0: uint8;
+  var_1: uint8;
+}
+
+table SmallStructs {
+  small_structs: [JustSmallStruct];
+}
+
+root_type SmallStructs;