Optimize String::push_byte()

```
test new_push_byte ... bench:      6985 ns/iter (+/- 487) = 17 MB/s
test old_push_byte ... bench:     19335 ns/iter (+/- 1368) = 6 MB/s
```

```rust
extern crate test;
use test::Bencher;

static TEXT: &'static str = "\
    Unicode est un standard informatique qui permet des échanges \
    de textes dans différentes langues, à un niveau mondial.";

#[bench]
fn old_push_byte(bencher: &mut Bencher) {
    bencher.bytes = TEXT.len() as u64;
    bencher.iter(|| {
        let mut new = String::new();
        for b in TEXT.bytes() {
            unsafe { new.as_mut_vec().push_all([b]) }
        }
    })
}

#[bench]
fn new_push_byte(bencher: &mut Bencher) {
    bencher.bytes = TEXT.len() as u64;
    bencher.iter(|| {
        let mut new = String::new();
        for b in TEXT.bytes() {
            unsafe { new.as_mut_vec().push(b) }
        }
    })
}
```
doc: --version no longer displays the host triple
mk: Fix bootstrapping the nightly builds

The stage0 compiler for a non-CFG_BUILD architecture needs to have the new
`-C extra-filename` argument passed.
auto merge of #15465 : SimonSapin/rust/patch-4, r=alexcrichton

`Vec::push_all` with a length 1 slice seems to have significant overhead compared to `Vec::push`.

```
test new_push_byte ... bench:      6985 ns/iter (+/- 487) = 17 MB/s
test old_push_byte ... bench:     19335 ns/iter (+/- 1368) = 6 MB/s
```

```rust
extern crate test;
use test::Bencher;

static TEXT: &'static str = "\
    Unicode est un standard informatique qui permet des échanges \
    de textes dans différentes langues, à un niveau mondial.";

#[bench]
fn old_push_byte(bencher: &mut Bencher) {
    bencher.bytes = TEXT.len() as u64;
    bencher.iter(|| {
        let mut new = String::new();
        for b in TEXT.bytes() {
            unsafe { new.as_mut_vec().push_all([b]) }
        }
    })
}

#[bench]
fn new_push_byte(bencher: &mut Bencher) {
    bencher.bytes = TEXT.len() as u64;
    bencher.iter(|| {
        let mut new = String::new();
        for b in TEXT.bytes() {
            unsafe { new.as_mut_vec().push(b) }
        }
    })
}
```
auto merge of #15467 : brson/rust/guideversion, r=alexcrichton
Add a test case for #12187, which appears to have been fixed

Closes #12187.
Remove remainders from when booleans were i8
Store booleans as i8 in memory to improve optimizations by LLVM

LLVM doesn't really like types with a bit-width that isn't a multiple of
8 and disable various optimizations if it encounters such types used
with loads/stores. OTOH, booleans must be represented as i1 when used as
SSA values. To get the best results, we must use i1 for SSA values, and
i8 when storing the value to memory.

By using range asserts on loads, LLVM can eliminate the required
zero-extend and truncate operations.

Fixes #15203
auto merge of #15486 : jakub-/rust/test-case-12187, r=alexcrichton

Closes #12187.
Properly bind nested pattern bindings when there's more than one

Fixes #15488.
auto merge of #15481 : alexcrichton/rust/fix-nightly, r=luqmana

The stage0 compiler for a non-CFG_BUILD architecture needs to have the new
`-C extra-filename` argument passed.
auto merge of #15464 : dotdash/rust/bool_stores, r=pcwalton

LLVM doesn't handle i1 value in allocas/memory very well and skips a number of optimizations if it hits it. So we have to do the same thing that Clang does, using i1 for SSA values, but storing i8 in memory.

Fixes #15203.
Implemented Decodable/Encodable for Cell and RefCell. Fixes #15395

Updated PR with fixme and test

Updated PR with fixme and test
auto merge of #15411 : mitchmindtree/rust/master, r=alexcrichton

I ran `make check` and everything went smoothly. I also tested `#[deriving(Decodable, Encodable)]` on a struct containing both Cell<T> and RefCell<T> and everything now seems to work fine.
manual: Fix typo.

Also removes redundant newline in code block.
auto merge of #15489 : jakub-/rust/issue-15488, r=pcwalton

Fixes #15488.
auto merge of #15496 : ruud-v-a/rust/patch-1, r=huonw

Also removes redundant newline in code block.