…orial, r=cmr

Work on #13287 

This is not ready for a merge yet, but I wanted to get some eyes on what I have done so far.

As of right now, all references in the text to managed boxes or pointers are removed. Code associated with those specific sections of text have likewise been altered. I also removed all references to managed closures.

There is a small change I would like to add to the work done in 3137cd5, on the new lines 1495 and 1496, I would like to change those values to 10 and 20. I did the same in a later change on lines 1596 and 1508.

There are still bits of sample code that use managed pointers and the sigil @. Those are next on my list to remove, but I wanted to have the outstanding changes reviewed first. The uses of @ in the code samples are a bit more embedded, and I will need to be more careful changing them as to not change the purpose of the code examples.

I ensured that make check still passes, although I'm not sure if that actually tests the code in tutorial.md.

One issues I ran into, and tried to avoid, was that `tutorial.md` is formatted with a nice column limit. I was unsure how this was enforced, so wherever I edited a line, I did my best to keep edits on the line they previously existed on. As such, the plain text of `tutorial.md` looks a bit strange as I've left it, and I will clean that up as suggested. The rendered markdown output should not be affected.

Fixes #13287
Completes work started in #13298

Removed all reference to and sample code using managed boxes/pointers.

Don't lint unnamed consts and nested items within functions in `missing_docs_in_private_items`

With this change we no longer require doc comments for `const _: ()` items as well as nested items in functions or other bodies. In both of those cases, rustdoc generates no documentation even with `--document-private-items`.

Fixes rust-lang#13427 (first commit)
Fixes rust-lang#13298 (second commit)
cc rust-lang/rust-clippy#5736 (comment)

changelog: [`missing_docs_in_private_items`]: avoid linting in more cases where rustdoc generates no documentation