Fix #430

Replace fold/reduce over Result::and with collect

Not only is this tidier, but it avoids making these iterators
Rayon-specific.

Use a HashSet for detecting orphan pages

This offers a big performance bump for large sites:

  small-kb (100 pages)
    before: 530ms
     after: 500ms

  medium-kb (1000 pages)
    before: 2.9s
     after: 1.5s

  huge-kb (10,000 pages)
    before: 150s
     after:   9.1s

Fix #383

Closes #418

Closes #388

Closes #417

Closes #403

Change highlighting to not include duplicated background colors

Fix #412

Fix typo

Next version

Update pagination for test_site posts

Updates docs to match proper toml syntax.

link_checker: Set Accept header

Fix rendering benchmarks.

As mentioned in #381, crates.io 404's any request without an Accept:
text/html header.  It 200's any request with one, but at least
false-successes don't prevent checking any other links.

This also makes it easier to add a custom User-Agent if desired.

rustfmt and fix a clippy nit (unnecessary return) while I'm here.

Improve summary handling

As mentioned in #376

Push summary handling into Markdown parsing, identifying the presence
of one by giving its length in the rendered markup.

Hopefully a better fix for #376.