[Swift] Moves capacity outside of Storage (#8650)

- Cleans up capacity usage within the lib and moves it outside of the Storage

-  Use overflow operators

tests/swift/Tests/Flatbuffers/ByteBufferTests.swift

@@ -42,7 +42,7 @@ final class ByteBufferTests: XCTestCase {
     let byteBuffer = ByteBuffer(data: ptr)
     byteBuffer.withUnsafeBytes { memory in
       ptr.withUnsafeBytes { ptr in
-        XCTAssertEqual(memory.baseAddress!, ptr)
+        XCTAssertEqual(memory.baseAddress!, ptr.baseAddress!)
       }
     }
   }