add "(see below)" to a reference to a new concept

This is because I observed someone reading the tutorial who thought they'd
missed something when they got to the mention of variable bindings.

This patch doesn't reflow the paragraphs so that you can see the semantic
change that I made, and a subsequent patch will reflow this paragraph.

Fixes https://github.com/rust-lang/rust/issues/13570.
reflow with emacs fill-paragraph (fill-column 71)
librustc: Remove cross borrowing from mutable `Box`es to `&mut`.

This will break code like:

    fn f(x: &mut int) {}

    let mut a = box 1i;
    f(a);

Change it to:

    fn f(x: &mut int) {}

    let mut a = box 1i;
    f(&mut *a);

RFC 33; issue #10504.

[breaking-change]
Guide: install instructions
fix feedback from @huonw
thanks @sfackler
Guide: Hello, world!

Let's write our first Rust program!
auto merge of #15171 : pcwalton/rust/remove-cross-borrowing, r=brson

This will break code like:

    fn f(x: &mut int) {}

    let mut a = box 1i;
    f(a);

Change it to:

    fn f(x: &mut int) {}

    let mut a = box 1i;
    f(&mut *a);

RFC 33; issue #10504.

[breaking-change]

r? @brson
Add extra ignore directives.
tidy macro just a bit
revive old commented-out test cases as ignored test cases for hygiene
comments only
enrich and rename crate_idents test case
add unit test for match var hygiene
remove unneeded visit_ty methods
remove unneccessary pubs, shorten names
remove misleading and unnecessary underscores
don't expand subexprs of for loop, just re-expand whole thing.

Fixes #15167
added compile-fail test for 15167
make macros hygienic