Remove entry_bcx from FunctionContext

We no longer need to refer to the entry block from arbitrary places, so
we can drop it from FunctionContext.
move lifetime suggestion note to after the error

Closes #15433.
Update RELEASES.txt

Fix Typos
Fix #15453
Improved example in the Guide

Fixes https://github.com/rust-lang/rust/issues/15452
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
auto merge of #15439 : dotdash/rust/remove_entry_bcx, r=pcwalton

We no longer need to refer to the entry block from arbitrary places, so
we can drop it from FunctionContext.
auto merge of #15441 : P1start/rust/lifetime-suggest, r=pcwalton

Closes #15433.
Register snapshot.
auto merge of #15472 : luqmana/rust/snaps, r=alexcrichton
auto merge of #15447 : patrickyevsukov/rust/patch-1, r=alexcrichton

Fix Typos
auto merge of #15454 : jakub-/rust/15453, r=huonw

I forget we now have byte string literals.
auto merge of #15456 : aochagavia/rust/guide, r=alexcrichton

Fixes https://github.com/rust-lang/rust/issues/15452
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