Verify transfer lib more thoroughly

[QGarchery]

I also wonder if we want some small unit tests, like:

• safeTransfer and safeTransferFrom revert if balance not enough.
• safeTransferFrom reverts if not authorized.
• safeTransfer does not revert if balance is enough.

Originally posted by @jhoenicke in #321 (review)

[QGarchery]

Done in #346