Support anyhow crate

[hiratara]

Hi,

I want to use lambda_calculus and https://github.com/dtolnay/anyhow crates in my app and wrote the following codes:

fn main() -> anyhow::Result<()> {
    let mut expr = parse(&"(λm.λn.λs.λz. m s (n s z)) (λs.λz. z) (λs.λz. s z)", Classic)?;
    ...
}

But it causes the error:

error[E0277]: the trait bound `ParseError: std::error::Error` is not satisfied
 --> src/main.rs:4:89
  |
4 |     let mut expr = parse(&"(λm.λn.λs.λz. m s (n s z)) (λs.λz. z) (λs.λz. s z)", Classic)?;
  |                                                                                         ^ the trait `std::error::Error` is not implemented for `ParseError`
  |
  = help: the following other types implement trait `FromResidual<R>`:
            <Result<T, F> as FromResidual<Result<Infallible, E>>>
            <Result<T, F> as FromResidual<Yeet<E>>>
  = note: required for `anyhow::Error` to implement `From<ParseError>`
  = note: required for `Result<(), anyhow::Error>` to implement `FromResidual<Result<Infallible, ParseError>>`

This PR implements the Error trait for ParseError and allows it to be converted into anyhow::Error .

I don't feel the need to convert to anyhow::Error for the TermError, but I implemented it just in case.

Note that I'd better use the https://github.com/dtolnay/thiserror trait for this purpose, but this crate should not depend on other crates, so I didn't use it.

[ljedrz]

Thanks! I agree, this is a very useful addition; just adjusting one error message and it's good to go - I will cut a release afterwards.