Move parser to separate crate?

[Lizzie Crowdagger]

It could make sense to move the parser part to a different crate (e.g. crowbook-parser).

Pros:

• would make the code more reusable (since it is build on pulldown-cmark, might not make most sense, but the enum-based parser might be easier to use by some people?)
• some additional feature (e.g. superscript) can require modification of parser code, it would break compatibility, which block for a 1.0 release (with this crowbook could be 1.0 even if crowbook-parser is still 0.x)

Cons:

• might not be that obvious to do ;
• is there really a need to that much crowbook-xxx libraries?

[Lizzie Crowdagger]

Actually probably not a good idea since Token needs to be exported anyway somehow