fix/vec_to_bytes_le

[Draply]

Check List

• Don't forget to squash commits into meaningful chunks before merging
• Check every test passed.
• Did you split imports into std and custom parts?
• Have you updated the consts in ORN?
• Have you run ORN with the latest version?
• Check your commit messages.
• Have you added meaningful comments?
• Don't forget to optimize gas cost!
  • Calling vector::length() multiple times is expensive (e.g. in loops), save it to a variable instead.
  • Consider using consts for things that can be pre-computed.
  • pow(2, x) => (1 << x+1)
  • inline short functions to save gas.
  • Find all vector::empty() in all files and replace them with the first append
  • Use vector::*_reverse_*() version to save gas.
• Check Visibility of functions