Removed references to managed boxes/pointers from the tutorial text. Code
examples in relevant sections were similarly altered.
Removed all references to managed closures.
std: Document builtin syntax extensions

These syntax extensions need a place to be documented, and this starts passing a
`--cfg dox` parameter to `rustdoc` when building and testing documentation in
order to document macros so that they have no effect on the compiled crate, but
only documentation.

Closes #5605
Add fetch_and, fetch_or, fetch_xor to AtomicInt, AtomicUint
stop asking LLVM to null-terminate strings

This was missed when dropping the null-termination from our string
types. An explicit null byte can still be placed anywhere in a string if
desired, but there's no reason to stick one at the end of every string
constant.
manual: update the note about reserved attributes
manual: many fixes

Mostly around attributes and language items.
etc: attempts at fixing editor highlighting for new attribute syntax
auto merge of #12964 : gereeter/rust/extended-atomics, r=alexcrichton

Fixes #12949.
manual: update style
manual: fix attribute ebnf
manual: further attribute clarifications

- Sort attributes lexicographically
- Use new attribute syntax consistently
- Fix wording describing new attribute syntax
- Expand section on `repr`
manual: refine ECMA-335 reference for attributes
auto merge of #13255 : alexcrichton/rust/issue-5605, r=huonw

These syntax extensions need a place to be documented, and this starts passing a
`--cfg dox` parameter to `rustdoc` when building and testing documentation in
order to document macros so that they have no effect on the compiled crate, but
only documentation.

Closes #5605
manual: fix a broken example
auto merge of #13207 : cmr/rust/attrib-reference, r=kballard
rustc: feature-gate `concat_idents!`.

concat_idents! is not as useful as it could be, due to macros only being
allowed in limited places, and hygiene, so lets feature gate it until we
make a decision about it.

cc #13294
auto merge of #13291 : thestinger/rust/no_null, r=alexcrichton

This was missed when dropping the null-termination from our string
types. An explicit null byte can still be placed anywhere in a string if
desired, but there's no reason to stick one at the end of every string
constant.
auto merge of #13295 : huonw/rust/gate-concat-idents, r=alexcrichton

rustc: feature-gate `concat_idents!`.

concat_idents! is not as useful as it could be, due to macros only being
allowed in limited places, and hygiene, so lets feature gate it until we
make a decision about it.

cc #13294
auto merge of #13298 : ckendell/rust/remove_managed_pointers_from_tutorial, 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.