Add examples of working with other libraries

[brendanzab]

It would be nice to show how codespan can be used in conjunction with other libraries. Some ones we might want to show are:

Candidate libraries

Incremental Compilation

We get a number of requests (#172, #179) about Salsa, so we could do with a simple example showing how we recommend using it with codespan-reporting!

• [ ] salsa

Parsers

Many people get started building languages using parser generators, so showing them how to get codespan hooked up to these would be very useful!

• [ ] LALRPOP
• [ ] combine
• [ ] Nom
• [ ] Pest (from memory this is hard-coded to use codemap, so I'm not sure if it is possible))
• [x] peg

Pretty printing

It would be nice to have an example of pretty printing types, with source code highlighting, and potentially nifty things like type diffs.

• [ ] pretty

[Stupremee]

I'm currently working on a project that uses codespan-reporting and salsa.

[brendanzab]

Oh wow, this looks like a nice thing to learn from in general!

[Kixiron]

I made a minimal example of using salsa & codespan-reporting to get on-demand computation (so you only calculate spans and stuff for the errors you actually emit)