Move printing to Display and parsing to a new trait, FromOp.

[rosekunkel]

This fixes #83 by replacing Language::display_op with OpStr::fmt Display. It also moves Language::from_op_str to OpStr::from_str FromOp::from_op. Moving these two functions to a separate trait eliminates the need to give them default implementations which panic. define_language! has been updated to automatically implement OpStr Display and FromOp, so the transition should be seamless for people using the macro.

Additionally, this pull request adds a dependency on thiserror, a crate which provides a derive macro for std::error::Error, as this refactoring replaces several instances of String being used as an error type with structured error types implementing Error.

Let me know if there's anything that should be adjusted!

[mwillsey]

Awesome! There's a lot of good stuff here. A couple thoughts:

• Why not just have Language implement Display? We could still have a separate parsing trait.
• I'm not ready for a thiserror dependency just yet, especially since we just have one error.

[mwillsey]

Ah, I see your thiserror changes are actually pretty widespread. I guess it's worth keeping then. thiserror is the future after all!

[rosekunkel]

Good questions! Re: Display, do you mean that we should mandate that if a Language implements Display, it should just display the name of the operator?

As I've been thinking about this more, I'm wondering whether we really want to be using strings at all. Right now the way these string functions are used is for constructing and parsing sexps, so maybe it would make more sense if we had ToSexp and FromSexp traits instead.

Also, re: thiserror, it's just a convenience macro, so if we want to remove the dependency later we can just implement Error manually instead.

[mwillsey]

Yes, Display just prints the operator.

That's true, it's mostly used to print sexps, but there are couple places where it's nice to just print the operators, like in Dot.

[rosekunkel]

Ok, I think that makes sense. I'll rework it into a trait for parsing and Display for printing the operator.

[rosekunkel]

@mwillsey I made the changes you suggested -- let me know if there's anything else I should change.

[mwillsey]

Nice work, thanks!