Review panics and todos in clar2wasm

[obycode]

There are some panics (usually from an expect) in the code. These are currently only for things that should never happen, but since it is easy to just return an error instead of panicking, let's always do that, and never panic. The only places where panics should remain would be for calls with literal arguments that can be statically verified that the panic will never occur, for example ClarityName::try_from("reporter").expect("BUG: valid string representation").

We also need to review any places where todo items were left in the code and ensure that an issue is opened for each of these items.