Add a few new free functions to alloc::rc for manipulating
uniquely-owned Rc values. is_unique() can be used to test if the Rc is
uniquely-owned, try_unwrap() can remove the value from a uniquely-owned
Rc, and get_mut() can return a &mut for a uniquely-owned Rc.

These are all free functions, because smart pointers should avoid having
methods when possible. They can't be static methods because UFCS will
remove that distinction. I think we should probably change downgrade()
and make_unique() into free functions as well, but that's out of scope.

A quick and dirty fix for #15036 until we get serious decoder reform.

Right now it is impossible for a Decodable to signal a decode error,
for example if it has only finitely many allowed values, is a string
which must be encoded a certain way, needs a valid checksum, etc. For
example in the libuuid implementation of Decodable an Option is
unwrapped, meaning that a decode of a malformed UUID will cause the
task to fail.

Since this adds a method to the `Decoder` trait, all users will need
to update their implementations to add it. The strategy used for the
current implementations for JSON and EBML is to add a new entry to
the error enum `ApplicationError(String)` which stores the string
provided to `.error()`.

[breaking-change]

When generating a unique symbol for things like closures or glue_drop,
we call token::gensym() to create a crate-unique Name. Recently, Name
changed its Show impl so it no longer prints as a number. This caused
symbols like glue_drop:1542 to become glue_drop:"glue_drop"(1542), or in
mangled form, glue_drop.$x22glue_drop$x22$LP$1542$RP$.

Errors can be printed with {}, printing with {:?} does not work very
well.

Not actually related to this PR, but it came up when running the tests
and now is as good a time to fix it as any.

This was motivated by a desire to remove allocation in the common
pattern of

    let old = key.replace(None)
    do_something();
    key.replace(old);

This also switched the map representation from a Vec to a TreeMap. A Vec
may be reasonable if there's only a couple TLD keys, but a TreeMap
provides better behavior as the number of keys increases.

Like the Vec, this TreeMap implementation does not shrink the container
when a value is removed. Unlike Vec, this TreeMap implementation cannot
reuse an empty node for a different key. Therefore any key that has been
inserted into the TLD at least once will continue to take up space in
the Map until the task ends. The expectation is that the majority of
keys that are inserted into TLD will be expected to have a value for
most of the rest of the task's lifetime. If this assumption is wrong,
there are two reasonable ways to fix this that could be implemented in
the future:

1. Provide an API call to either remove a specific key from the TLD and
   destruct its node (e.g. `remove()`), or instead to explicitly clean
   up all currently-empty nodes in the map (e.g. `compact()`). This is
   simple, but requires the user to explicitly call it.
2. Keep track of the number of empty nodes in the map and when the map
   is mutated (via `replace()`), if the number of empty nodes passes
   some threshold, compact it automatically. Alternatively, whenever a
   new key is inserted that hasn't been used before, compact the map at
   that point.

---

Benchmarks:

I ran 3 benchmarks. tld_replace_none just replaces the tld key with None
repeatedly. tld_replace_some replaces it with Some repeatedly. And
tld_replace_none_some simulates the common behavior of replacing with
None, then replacing with the previous value again (which was a Some).

Old implementation:

    test tld_replace_none      ... bench:        20 ns/iter (+/- 0)
    test tld_replace_none_some ... bench:        77 ns/iter (+/- 4)
    test tld_replace_some      ... bench:        57 ns/iter (+/- 2)

New implementation:

    test tld_replace_none      ... bench:        11 ns/iter (+/- 0)
    test tld_replace_none_some ... bench:        23 ns/iter (+/- 0)
    test tld_replace_some      ... bench:        12 ns/iter (+/- 0)

Closes #16097 (fix variable name in tutorial)
Closes #16100 (More defailbloating)
Closes #16104 (Fix deprecation commment on `core::cmp::lexical_ordering`)
Closes #16105 (fix formatting in pointer guide table)
Closes #16107 (remove serialize::ebml, add librbml)
Closes #16108 (Fix heading levels in pointer guide)
Closes #16109 (rustrt: Don't conditionally init the at_exit QUEUE)
Closes #16111 (hexfloat: Deprecate to move out of the repo)
Closes #16113 (Add examples for GenericPath methods.)
Closes #16115 (Byte literals!)
Closes #16116 (Add a non-regression test for issue #8372)
Closes #16120 (Deprecate semver)
Closes #16124 (Deprecate uuid)
Closes #16126 (Deprecate fourcc)
Closes #16127 (Remove incorrect example)
Closes #16129 (Add note about production deployments.)
Closes #16131 (librustc: Don't ICE when trying to subst regions in destructor call.)
Closes #16133 (librustc: Don't ICE with struct exprs where the name is not a valid struct.)
Closes #16136 (Implement slice::Vector for Option<T> and CVec<T>)
Closes #16137 (alloc, arena, test, url, uuid: Elide lifetimes.)

Signed-off-by: NOGINO Masanori <masanori.ogino@gmail.com>

Fixes #11511.

This now works because of elision.

Fixes #16117

This deprecates the hexfloat library as it is now located in the rust-lang
organization as a cargo package

This initialization should happen unconditionally, but the rtassert! macro is
gated on the ENFORCE_SANITY define

Closes #16106

Our implementation of ebml has diverged from the standard in order
to better serve the needs of the compiler, so it doesn't make much
sense to call what we have ebml anyore. Furthermore, our implementation
is pretty crufty, and should eventually be rewritten into a format
that better suits the needs of the compiler. This patch factors out
serialize::ebml into librbml, otherwise known as the Really Bad
Markup Language. This is a stopgap library that shouldn't be used
by end users, and will eventually be replaced by something better.

[breaking-change]

For consistency, just because `fail_` has it.

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
```

Smaller text size.

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

In order to prevent users from having to manually implement Hash and Ord for
bitflags types, this commit derives these traits automatically.

This breaks code that has manually implemented any of these traits for types
created by the bitflags! macro. Change this code by removing implementations
of these traits.

[breaking-change]