Pull out pure Rust parser into it's own crate

[xfbs]

Hey @stepancheg,

Awesome work here!

I would like to use some of this for a project. Basically, I would like to be able to parse protocol buffer files and do some operations on the AST. A few use cases:

• Doing some validation of protocol buffer files
• Dome some linting (parsing the AST, and then writing it back out)
• Displaying information in a user interface (for example, it would be nice to have access to comments)

For context, this work is part of buffrs.

My idea was that it might be possible to pull out the pure Rust parser from protobuf-parse and expose it's internal AST directly — using the FileDescriptor types is not all that useful because they lack things such as line information.

What do you think — if I started doing some work on this, would you support it? I do not want to create any extra work for you, I was just thinking that it might be good to build this in a way that others can use it, too 😄

Cheers,
Patrick