Allow manual implementations of built-in traits

[RFC #3]

cc #13231
librustc: Remove `~"string"` and `&"string"` from the language
Add serialization support for StrBuf

- implement Encodable and Decodable for StrBuf
- implement to_json for StrBuf
rustdoc tutorial: grammar change in list comments
Fixed outdated note in the tutorial
auto merge of #13875 : hjr3/rust/serialize-strbuf, r=alexcrichton

- implement Encodable and Decodable for StrBuf
- implement to_json for StrBuf
Add more comprehensive unit tests for SIMD binops
BigUint always use u64 as the internal machine unsigned integer

This change allow a speedup of ~1.5 on shootout-pidigits on a i32
system.  `DoubleBigDigit` is used to abstract the internal
unsigned integer used in computation to simplity future
architecture specialization.

`BigDigit::from_uint` and `BigDigit::to_uint` become
`BigDigit::from_doublebigdigit` and `BigDigit::to_doublebigdigit`.

[breaking-change]
auto merge of #13878 : brutal-chaos/rust/tutorial_grammar, r=alexcrichton
auto merge of #13881 : aochagavia/rust/pr, r=alexcrichton
Allow built-in traits to be derived

[RFC #3]

cc #13231
remove leftover obsolete string literals
fix error message for obsolete &"foo" literal
auto merge of #13877 : thestinger/rust/de-tilde-str-vec, r=alexcrichton
Fix a/an typos
Add debug_assert and debug_assert_eq macros

I also switched some `assert!` calls over to `debug_assert!`.

Closes #12049.

RFC: 0015-assert
Remove useless assert! case
auto merge of #13886 : japaric/rust/fix-an-typos, r=alexcrichton

Found the first one in the rust reference docs. I was going to submit a PR with one fix, but figured I could look for more... This is the result.
auto merge of #13789 : sfackler/rust/debug-assert, r=pcwalton

I switched the `assert!` calls in `RefCell` over to `debug_assert!`.
There are probably other instances that should be converted as well, but
I couldn't think of any off the top of my head.

RFC: 0015-assert
Implement ToJson for Vec<T>

Now that ~[T] is obsolete, we need to allow to_json() to work for
vectors.