add assignment for converting errors with `From<T>` and `Into<T>`

[miguelraz]

and using the ? to get the neat UX

cc @listochkin , who we were talking about this

• buildup to the thiserror crate usage:

#[derive(Debug, thiserror::Error)]
pub enum ParseError {
    #[error("Not enough operands")]
    NotEnoughOperands,
    #[error("Not a number {0}")]
    NotANumber(String),
}

#[derive(Debug, thiserror::Error)]
pub enum CalcError {
    #[error("{0}")]
    ParseError(#[from] ParseError),
    #[error("{0}")]
    EvalError(#[from] EvalError),
}

• show the IDE "expand macro recursively" and "implement all fields"