Add ERC-1155, multisigs, and flipper

[Guillaume Claret]

Add the following contracts:

• https://github.com/formal-land/ink/blob/master/integration-tests/erc1155/lib.rs
• https://github.com/formal-land/ink/tree/master/integration-tests/multisig
• https://github.com/formal-land/ink/tree/master/integration-tests/flipper

to the folder https://github.com/formal-land/coq-of-rust/tree/main/contracts/source The code of the contracts should be stripped of the tests.

It should pass the tests of the CI. If this does not work, modify https://github.com/formal-land/coq-of-rust/blob/main/contracts/source_to_generated.py or https://github.com/formal-land/coq-of-rust/blob/main/contracts/template/src/storage.rs so that the generated Rust type-checks.

[Guillaume Claret]

Not a priority anymore.