Add EDN pretty printer

[ncalexan]

This ticket tracks adding a pretty-printer (potentially, implementing std::fmt::Display) for edn::types::Value.

If I was to do this, I'd start by understanding how serde achieves this, and then whether there's a Rust implementation of Wadler's pretty-printer combinators.

[ncalexan]

This is a [good first bug] if you have strong development skills but not necessarily strong Rust skills.

[ncalexan]

@jsantell can you bump this up your priority queue? It's frustrating having 4k of output on one line when dumping these databases :) Thanks!

[ncalexan]

We have a lot of input and output that looks like [[:db/add ...] [:db/add ...]]. It would be nice to give columnar views easily, using (for example) https://github.com/BurntSushi/tabwriter. By columnar, I mean like:

[[:db/add e1      attribute1    value1]
 [:db/add entity2 attr2         value2]]

both pretty-printed and tab-aligned in some situations. Just a nice to have, might not be useful in general.

[victorporof]

As discussed, I'll take this.

[ncalexan]

I see Marwes/pretty.rs#26 tracking adding some simple documentation. The examples and tests are more than enough to complete that issue, which would be a great contribution to upstream...