feat: MPL Core NFTs

[IaroslavMazur]

refactor: renounce the NftCollectionData
refactor: renounce nft.rs

test: adapt for MPL Core NFTs

docs: minor wording enhancements

Closes #270.

Notes:

• Thanks to the single-account structure for both the Assets & Collections in MPL Core, switching to this newer NFT standard helped us greatly reduce
  • the number of accounts our Ixs require,
  • the number of CPIs we need to perform in our program,
  • the code volume, complexity and size, as well as
  • the cost of creating a Stream NFT (supported by our end users) and
  • the cost of deploying our Lockup program to Mainnet (aka the rent cost)
• Because the MPL Core's BaseCollectionV1 tracks its total supply itself, we could delete the NftCollectionData.