RFE: add API for parsing licenses

[ignatenkobrain]

It would be really awesome if there would be API after crate parsed license, so it could be hooked to some tools (e.g. in rust2rpm in my case).

[wking]

I don't have enough Rust to understand the implementation, but there's some AST discussion in #5. Maybe look that over?

[Nemo157]

I have just taken a quick look at #5, for some reason it's not compiling at the moment. It looks ok, but I have no experience with lalrpop and really don't like the 2000+ line generated file being committed to the repository.

I'm currently thinking of redoing this from scratch using a nom based parser (because I'm somewhat familiar with nom), it's a very simple language to parse so I don't think losing out on the existing parser work is very bad.

For my reference, here's a couple of projects used by other package managers that will likely be useful sources of test data:

• https://github.com/composer/spdx-licenses
• https://github.com/jslicense/spdx-expression-parse.js