Implement RFC#28: Add PartialOrd::partial_cmp

I ended up altering the semantics of Json's PartialOrd implementation.
It used to be the case that Null < Null, but I can't think of any reason
for an ordering other than the default one so I just switched it over to
using the derived implementation.

This also fixes broken `PartialOrd` implementations for `Vec` and
`TreeMap`.

RFC: 0028-partial-cmp
auto merge of #15030 : sfackler/rust/partial-cmp, r=huonw

I ended up altering the semantics of Json's PartialOrd implementation.
It used to be the case that Null < Null, but I can't think of any reason
for an ordering other than the default one so I just switched it over to
using the derived implementation.

This also fixes broken `PartialOrd` implementations for `Vec` and
`TreeMap`.

# Note
This isn't ready to merge yet since libcore tests are broken as you end up with 2 versions of `Option`. The rest should be reviewable though.

RFC: 0028-partial-cmp
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