Use a macro in lang_items to remove duplication.

This should make maintenance of lang items simpler and also reduces the
line count by about 201 lines.
Check crate root for docs in missing_doc lint.

Because the root module isn't actually an item, we need to do some hackish
handling of it.

Closes #10656.
auto merge of #10665 : cmr/rust/doc_lint, r=alexcrichton

Because the root module isn't actually an item, we need to do some hackish
handling of it.

Closes #10656.
auto merge of #10832 : chris-morgan/rust/let's-lop-lang-item-line-count, r=alexcrichton

This should make maintenance of lang items simpler and also reduces the
line count by about 201 lines.