Escrow smart contract update

Renames the escrow contract creation to the escrow ~~account~~ wallet creation to avoid confusion with the escrow contract itself.
Reduces Action & Event enums by removing obvious contract_id field identifiers from them: https://github.com/gear-tech/apps/blob/f21c37f832b356d823f1b9ee1b91e07769da4771/escrow/io/src/lib.rs#L17-L28
Adds metadata & the meta_state function for reading escrow accounts info.
Adds the type alias for an account ID and changes its type from u128 to U256 for greater capacity, like in the example implementation of NFT: https://github.com/gear-tech/apps/blob/f21c37f832b356d823f1b9ee1b91e07769da4771/nft-example/src/lib.rs#L21
Rewrites utils for tests to make them more readable.
Minor changes in docs/comments/panic messages.

gear-tech / apps