Enable more optional clippy lints when building contracts

[jubnzv]

We could enable some optional clippy lints to find common errors when compiling ink! contracts.

Since smart contracts demand increased security requirements, it makes sense to add as much available security checks as possible, if it is feasible to get any reasonable results on ink! contracts.

My suggestion is to enable clippy lints to highlight some constructions that look suspicious in production contracts. It would be good to draw the additional attention to them:

1. panic! and panic! inside functions returning Result
2. todo!
3. unimplemented!
4. unreachable!

There is also the .unwrap() and .expect() lint that works for Option and Result types, but it would be too noisy if we add it.

By default, all these lints are suppressed. I suggest changing their warning level to Warn to allow them to raise non-critical warnings when building with cargo-contract. This might be achieved by changing a single line in cargo-contract, enabling us to reuse the existing infrastructure without extending ink_linting.