implement support for book parts

[mark-i-m]

Implements support for breaking the list of chapters in SUMMARY.md into parts. This is useful for really long books that (e.g. rustc-dev-guide).

Some notable things:

• I think this makes breaking changes to some public APIs of the mdbook crate.
• I did some refactoring on the SUMMARY.md parser that I think cleans things up a bit. In particular, I gave the parser the ability to lookahead 1 token, which got rid of some edge cases and prevents every combinator from having to figure out if they are missing their first token. The new convention is that if your combinator discovers it has gone too far, it puts back the last token it read.

Here is a sample of what it looks like:

[mark-i-m]

All tests are passing locally for me.

[terretta]

+1 -- looking for this.

Currently, mdbook knows about this SUMMARY.md syntax but doesn't leverage:
https://github.com/rust-lang/book/blob/2c942dc094f4ddcdc7aba7564f80782801197c99/second-edition/src/SUMMARY.md#basic-rust-literacy

Using ## to indicate parts in SUMMARY.md is used at GitBook as well. This PR allows rendering of a matching Nav, while the current code drops the part names entirely.

‌# Summary​

## Use headings to create page groups like this one​

* [First page's title](page1/README.md)    
    * [Some child page](page1/page1-1.md)    
    * [Some other child page](part1/page1-2.md)
    
* [Second page's title](page2/README.md)    
    * [Some child page](page2/page2-1.md)    
    * [Some other child page](part2/page2-2.md)    
    
## A second-page group​

* [Yet another page](another-page.md)

GitBook doc: https://docs.gitbook.com/integrations/github/content-configuration#summary

[mark-i-m]

It's been a while -- I don't exactly remember, but I think I only implemented 1 level of headers in this PR...

[ehuss]

Thanks for your patience!

I can't quite see why summary chapters are split into separate Parts. Like, why couldn't headers be just another variant of SummaryItem and treated similarly to SummaryItem::Separator? Is there somewhere that structure is necessary?

Also, this will need some documentation (probably in https://github.com/rust-lang/mdBook/blob/master/book-example/src/format/summary.md).

[mark-i-m]

@ehuss I don't really remember why I did that; I don't think it occurred to me when I was modifying the summary parser. It doesn't look that hard to change, if you would like me to.

I can also update the documentation you pointed to. Does the comment on parse_summary look roughly like what you want in the documentation?

[ehuss]

I don't know if there will be any problems, but I would probably prefer using SummaryItem. The BookItem is designed to be recursive, but AFAICT the part titles are not. And, it doesn't really change the "structure" of the book, so I think it would be good to avoid changing the API in that way.

[mark-i-m]

@ehuss Done. I think it was a good idea, it reduced the diff somewhat.

[ehuss]

Thanks! I pushed a small tweak to the documentation wording.