librustc: Allow coercions through arrays.
Little typo fix

- ./hello_world which is not recognized on Windows fixed.
- .exe extension added.
- Little rewriting.
std: make sure HashMap from_iter uses random initialization by default

It turns out that HashMap's from_iter implementation was being
initialized without the sip keys being randomized. This adds
a custom default hasher that should avoid this potential vulnerability.
std: micro optimize Hash{Map,Set}::{new,with_capacity}
add example for from_byte() documenation
Add `utf16_units`

This deprecates `.to_utf16`. `x.to_utf16()` should be replaced by either
`x.utf16_units().collect::<Vec<u16>>()` (the type annotation may be optional), or
just `x.utf16_units()` directly, if it can be used in an iterator context.

Closes #14358

[breaking-change]
auto merge of #14613 : schmee/rust/utf16-iterator, r=huonw

Closes #14358.

~~The tests are not yet moved to `utf16_iter`, so this probably won't compile. I'm submitting this PR anyway so it can be reviewed and since it was mentioned in #14611.~~ EDIT: Tests now use `utf16_iter`.

This deprecates `.to_utf16`. `x.to_utf16()` should be replaced by either `x.utf16_iter().collect::<Vec<u16>>()` (the type annotation may be optional), or just `x.utf16_iter()` directly, if it can be used in an iterator context.

[breaking-change]

cc @huonw
Add `impl Show for &Show`

This makes printing a `Show` trait object much easier.
JSON cleanup

* Tried to make the code more idiomatic
* Renamed the `wr` field of the `Encoder` and `PrettyEncoder` structs to `writer`
* Replaced some `from_utf8` by `from_utf8_owned` to avoid unnecessary allocations
* Removed unnecessary `unsafe` code
Removed unnecessary Box
Add `encode` and `decode` shortcut functions

Now you can just use `json::encode` and `json::decode`, which is very
practical

**Deprecated `Encoder::str_encode` in favor of `json::encode`**

[breaking-change]
Implement FromStr for Json
Fix JSON documentation

Fixed some errors, removed some code examples and added usage of the
`encode` and `decode` functions.
Fix issues in libtest
Fix issue with tests
Fix issues in librustdoc
Implement ToJson for all tuples
auto merge of #15238 : aochagavia/rust/json, r=alexcrichton

### Breaking changes:

* **Removed unnecessary `box` from enum variant (`Object(Box<Object>)` becomes `Object(Object)`)**
* **Deprecated `Encoder::str_encode`**

### Other changes:

* Tried to make the code more idiomatic
* Renamed the `wr` field of the `Encoder` and `PrettyEncoder` structs to `writer`
* Replaced some `from_utf8` by `from_utf8_owned` to avoid unnecessary allocations
* Removed unnecessary `unsafe` code
* Added `encode` and `decode` shortcut functions
* Implemented `FromStr` for `Json`
* Implemented `ToJson` for tuples of arity up to 12
* Fixed some details in the documentation

### Questions

* ~~The `encode` shortcut function does the same as the `Encoder::str_encode` function. It seems wrong to me that two functions do exactly the same. Should we deprecate `Encoder::str_encode`?~~
* ~~Do we really want the ToJson trait for tuples? At the moment we have it for (), (A, B), (A, B, C). I would like to remove them.~~
* ~~We are using `String` as key in the `TreeMap` representing a `Json` object. It would be better to use `&str`, but this would require to annotate lots of lifetimes. Is there any easy solution for this?~~
* There is a lot of duplicated code (`PrettyEncoder` copies about 50 lines from `Encoder`). In an OO language this could be solved very elegantly by using inheritance and overriding. What can we do here to reduce the amount of boilerplate?

[breaking-change]
core: Reorganize slice module.

This is just moving things around so they are defined in a logical order.
core: Remove the unnecessary 'traits' module from 'slice'

This is only breaking for code that references the empty `slice::traits`
module.

[breaking-change]