CronCat errors macro

This package creates a procedural attribute macro that creates #[croncat_error], intended to be placed above the integrating contract's error enum, which is conventionally called ContractError.

Note: this macro should go above the usual Derive we see for this enum. The correct placement looks like this:

#[croncat_error]
#[derive(Error, Debug, PartialEq)]
pub enum ContractError {
  // variants here…
}

It will throw a compile-time error if it is not placed correctly.

The macro adds this enum variant:

#[error("CronCat error: {err:?}")]
  CronCatError {
  err: CronCatContractError
}

It also adds this logic:

impl From<CronCatContractError> for ContractError {
  fn from(error: CronCatContractError) -> Self {
    ContractError::CronCatError {
      err: error,
    }
  }
}

The above logic will allow helper functions in croncat-integration-utils to propagate errors in a manner consistent with the contract's errors.