doc: Monty Hall simulation for std::rand

A larger example for std::rand
doc: More efficient Monty Hall simulation
doc: use //! instead of /*! ... */ in std::rand
remove unused field CrateContext::uses_gc
Refactored syntax::fold.

Prior to this, the code there had a few issues:

- Default implementations inconsistently either had the prefix `noop_` or
  not.
- Some default methods where implemented in terms of a public noop function
  for user code to call, others where implemented directly on the trait
  and did not allow users of the trait to reuse the code.
- Some of the default implementations where private, and thus not reusable
  for other implementors.
- There where some bugs where default implementations called other default
  implementations directly, rather than to the underlying Folder, with the
  result of some AST nodes never being visited even if the user implemented that
  method. (For example, the current Folder never folded struct fields)

This commit solves this situation somewhat radically by making _all_
`fold_...` functions in the module into Folder methods, and implementing
them all in terms of public `noop_...` functions for other implementors to
call out to.

Some public functions had to be renamed to fit the new system, so this is a
breaking change.

[breaking-change]
Add a few more derivings to AST types
Port Rust to DragonFlyBSD

Not included are two required patches:

* LLVM: segmented stack support for DragonFly [1]

* jemalloc: simple configure patches

[1]: http://reviews.llvm.org/D4705
Update docs for TLS -> TLD

The correct terminology is Task-Local Data, or TLD. Task-Local Storage,
or TLS, is the old terminology that was abandoned because of the
confusion with Thread-Local Storage (TLS).
Deprecate the url crate.

The replacement is [rust-url](https://github.com/servo/rust-url),
which can be used with Cargo.

Fix #15874
Fix #10707
Close #10706
Close #10705
Close #8486
Fix trailing whitespace
Fix test suite for DragonFly
auto merge of #16076 : SimonSapin/rust/deprecate-url, r=alexcrichton

The replacement is [rust-url](https://github.com/servo/rust-url), which can be used with Cargo.

Fix #15874
Fix #10707
Close #10706
Close #10705
Close #8486
auto merge of #16090 : epdtry/rust/doesnt-use-gc, r=alexcrichton
fix variable name in tutorial
rustrt: Make begin_unwind take a single file/line pointer

Smaller text size.
Modify failure lang items to take less pointers.

Divide-by-zero before:

```
        leaq    "str\"str\"(1762)"(%rip), %rax
        movq    %rax, 16(%rsp)
        movq    $27, 24(%rsp)
        leaq    "str\"str\"(1542)"(%rip), %rax
        movq    %rax, (%rsp)
        movq    $19, 8(%rsp)
        leaq    16(%rsp), %rdi
        leaq    (%rsp), %rsi
        movl    $32, %edx
        callq   _ZN7failure5fail_20hc04408f955ce60aaqWjE@PLT
```

After:

```
        leaq    .Lconst(%rip), %rdi
        callq   _ZN7failure5fail_20haf918a97c8f7f2bfqWjE@PLT
```

Bounds check before:

```
        leaq    "str\"str\"(1542)"(%rip), %rax
        movq    %rax, 8(%rsp)
        movq    $19, 16(%rsp)
        leaq    8(%rsp), %rdi
        movl    $38, %esi
        movl    $1, %edx
        movl    $1, %ecx
        callq   _ZN7failure17fail_bounds_check20hf4bc3c69e96caf41RXjE@PLT
```

Bounds check after:

```
        leaq    .Lconst2(%rip), %rdi
        movl    $1, %esi
        movl    $1, %edx
        callq   _ZN7failure17fail_bounds_check20h5267276a537a7de22XjE@PLT
```

Size before:

21277995 librustc-4e7c5e5c.s

```
text       data
12554881   6089335
```

Size after:

21247617 librustc-4e7c5e5c.so

```
text       data
12518497   6095748
```
core: Add #[inline(never)] to failure functions

For consistency, just because `fail_` has it.
core: Fix failure doc comment
Fix deprecation commment on `core::cmp::lexical_ordering`
fix formatting in pointer guide table