[edn] Expose line/column/character span position information from parsed EDN streams

[ncalexan]

This ticket tracks including line/column/character position information with parsed edn::Value trees. It addresses #149 (comment); it is a pre-requisite, I think, for #168.

Concretely, I think we want a pair type like:

struct edn::ValueAndSpan {
  value: edn::Value,
  span: edn::Span,
}

which encapsulates a Span of some description coming out of the PEG EDN parser. Sadly, this means we'll need to rewrite or duplicate the edn::Value type to have edn::ValueAndSpan children. I could imagine something like:

• edn::Value<V>, where V parameterizes the children (so V = edn::Value is what we have now)

or something like

• edn::Value<S>, where S parameterizes the span (so S = () is what we have now).

We'll want to expose this new type, and probably convert to the existing type as well (just dropping all the spans).

This is pretty awkward -- better suggestions appreciated. This could be a good first bug if the patch doesn't try to change existing code too much (i.e., just adds new types and conversions into existing types).

[rnewman]

Why not just make edn::Value your ValueAndSpan?

Each fork of the enum would be (value, span). Callers would need to match with _ to discard the span, but that's OK: it forces them to think about it.

[rnewman]

That is: we already have a type for just the internal value — the internal value itself.

[ncalexan]

Why not just make edn::Value your ValueAndSpan?

Two reasons:

• it churns a lot of existing code, making this not a good first bug;
• because it makes programatically generating edn::Value difficult. You can introduce some NullSpan, I guess, but eventually you're going to want just "raw values".