ERC-721: Clear approval in burn()

[zgrannan]

Summary

• [n] y/n | Does it introduce breaking changes?
• [n] y/n | Is it dependant on the specific version of cargo-contract or pallet-contracts?

burn() should probably clear token approval for the token. Otherwise, the following sequence of events is possible:

1. Alice mints a token with id T
2. Alice grants approval to Bob for T
3. Alice burns the token
4. Frank mints the token, also with id T
5. Bob transfers the token to himself.

Mainly the concern would be that steps 1-3 could be performed by someone frontrunning, i.e. they see a pending transaction for step 4 in the mempool, and perform steps 1-3 first.

This PR updates the burn function to also clear the token approval, and adds a test for the described scenario above (with step 5 failing).

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have added an entry to CHANGELOG.md
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules