Add Pest Syntax to Ion Conversion

[almann]

It would be good to be able to use the Pest grammar definition to create other artifacts. For example, we could use the Pest grammar to generate LaTeX figures for the specification, and also derive things like a TextMate.

To make this easier to do, we should have a way to convert the Pest AST into Ion representation. This generalized representation can be used outside of Rust (e.g. a Python script) to make some of these non-critical path cases easier to write.

I propose we add a simple library to convert Pest to Ion and then a CLI that to do the conversion.