Now error messages highlight the error location

[CreepySkeleton]

A work on error reporting, as promised in #5 .

First of all, I must genuinely apologize for this long delay, the summer had happened to be more busy and eventful than I'd expected. I don't think something like that will happen this month, although you never know what's going to happen, don't you? Sorry again.

About PR:

It seems like the current (old) implementation drops Spans as soon as possible which was quite sensible: there was no way to use it so there was no need to carry needless stuff around. Worse yet: most of data structures are enums and one doesn't simply attach a piece of data to an enum. Basically, we have two ways:

• first is to attach this data to every variant, like

  pub enum Ty {
      Bool(Span),
      Vec(Span),
      Option(Span),
      OptionOption(Span),
      OptionVec(Span),
      Other(Span),
  }

  Easy to see, this is extremely cumbersome. Despite this, I had to use this approach for some enums
  (or just one... can't remember), mostly because I was forced to by a trait interface.

• second is to separate the kind and the metadata:

  pub struct Ty {
      span: Span,
      kind: TyKind
  }
  
  pub enum TyKind { Bool, Vec /* ... */ }

  The drawback here is there are a quite a lot of enums out here. I chose this approach, but slightly modified. There is struct Sp<T> in spanned module. It's essentially a wrapper around T which carries the span needed. For the sake of convince it implements Deref and some From traits.

Besides the Span problem mentioned, I've performed some refactoring, typically simplifying the code .

Most relevant changes are in src/*.rs files.

[TeXitoi]

Thanks for the contribution. I'll try to be fast on the review, but no promise.

Don't be sorry! We give our free time on these project, and thus we don't have any obligation.

[TeXitoi]

Great job! I love it! Really a better programming experience!

Some minor remarks, feel free to address/respond/ignore, that's mergeable as is!

Thanks again!

[TeXitoi]

Thanks! Now I must find time to do #217 and then publish v0.3.

[CreepySkeleton]

You should consider to publish v0.2.19 so people who have to stick to v0.2.x for one reason or another could make use of this improvement. Contrary wise, you may want not to do that to encourage people to move to v0.3.x and publish 0.2.19 only if/when requested. Anyway, you should know better.

[TeXitoi]

There is a lot to backport for that. Current master is already breaking 0.2.