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)
Guide: install instructions
fix feedback from @huonw
thanks @sfackler
Guide: Hello, world!

Let's write our first Rust program!
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This change starts denying `*T` in the parser. All code using `*T` should ensure
that the FFI call does indeed take `const T*` on the other side before renaming
the type to `*const T`.

Otherwise, all code can rename `*T` to `*const T`.

[breaking-change]
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
work around 15189 in test cases