Add note about string indexing.

Thanks @chris-morgan!
Be explicit with rustdoc.

I missed some annotations, and some were in a different style.
Add section about Str trait
librustc: Obsolete the old external crate renaming syntax.

Instead of `extern crate foo = bar`, write `extern crate bar as foo`.
Instead of `extern crate baz = "quux"`, write `extern crate "quux" as
baz`.

Closes #16461.

[breaking-change]
auto merge of #16824 : steveklabnik/rust/string_guide_improvements, r=alexcrichton

A few steps toward https://github.com/rust-lang/rust/issues/15994
auto merge of #17048 : pcwalton/rust/remove-old-import-renaming-syntax, r=brson

Instead of `extern crate foo = bar`, write `extern crate bar as foo`.
Instead of `extern crate baz = "quux"`, write `extern crate "quux" as
baz`.

Closes #16461.

[breaking-change]

r? @brson
auto merge of #17108 : steveklabnik/rust/explicitness, r=alexcrichton

I missed some annotations, and some were in a different style.
micro-optimize dynamic allocation alignment

Previously, some parts of this optimization were impossible because the
alignment passed to the free function was not correct. That was fully
fixed by #17012.

Closes #17092
implement sized deallocation

Closes #13994
auto merge of #17095 : thestinger/rust/alloc, r=alexcrichton

Previously, some parts of this optimization were impossible because the
alignment passed to the free function was not correct. That was fully
fixed by #17012.

Closes #17092