User could send nft to sendToCosmos and lose it

[yihuang]

crypto-org-chain/cronos#72

We find that user could call sendToCosmos with erc721 tokens, and the nft will end up bridged as a fungible token, with the same count as the token_id.
One possible solution is, erc721 actually have the onERC721Received to prevent user sending nft to unsupported contracts accidentally, but we are not using that because we are not calling the safeTransferFrom method on the token contract itself, but calling the one in SafeERC20 library.
So the proposal is in sendToCosmos we should call the safeTransferFrom on the token contract first, if it's not implemented we call the one in SafeERC20.

[yihuang]

yihuang/cronos@34bff59#diff-8d013786cc122121183707f7978f697504067b2cc7601bc8a1883498ed7c632a

This is a solution I came up with.