doc/guide-ffi: A few minor typo/language fixes

Signed-off-by: Anton Lofgren <alofgren@op5.com>
Add example for str replace() and MaybeOwned

  - for 3 implementations of into_maybe_owned()
  - is_slice()
  - is_owned()
Define a new setting g:rust_fold

g:rust_fold allows folding to be enabled. This lets the user turn on
folding without having to define autocommands.
Set softtabstop, textwidth, and optionally colorcolumn

Setting softtabstop makes <Del> delete 4 spaces as if it were a tab.

Setting textwidth allows comments to be wrapped automatically. It's set
at 80, which is the recommended line length for Rust programs. There are
suggestions that it should be 79, but our current style guide says 80 so
that's what we're matching.

A new setting g:rust_colorcolumn sets colorcolumn as well, to +1,101.
This indicates both the textwidth and the second stricter line length of
100 that our style guide lists.
Remove rust_colorcolumn, set textwidth to 99

The latest change to aturon/rust-guidelines states that lines must not
exceed 99 characters. This gets rid of the 80/100 split, so we don't
need to customize colorcolumn amymore.
Guide: match
Guide: looping
Guide: Standard input
librustc: Don't emit call for intrinsics instead just trans at callsite.
librustc: Remove old codepaths for creating intrinsic functions.
librustc: Update to reflect changes to how intrinsics are codegened.
Derive Clone for Command and StdioContainer
librustc: Translate input for transmute directly into dest.
libcore: Reexport a couple of widely-used low-level intrinsics to reduce
code bloat.

This didn't make a difference in any compile times that I saw, but it
fits what we're doing with `transmute` and seems prudent.
Use std::fmt::radix instead of to_str_radix.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Add range lint for float literals, fixing #10934
auto merge of #15566 : japaric/rust/command-clone, r=alexcrichton

Allows use cases like this one:

``` rust
use std::io::Command;

fn main() {
    let mut cmd = Command::new("ls");
    cmd.arg("-l");

    for &dir in ["a", "b", "c"].iter() {
        println!("{}", cmd.clone().arg(dir));
    }
}
```

Output:
```
ls '-l' 'a'
ls '-l' 'b'
ls '-l' 'c'
```
Without the `clone()`, you'll end up with:
```
ls '-l' 'a'
ls '-l' 'a' 'b'
ls '-l' 'a' 'b' 'c'
```

cc #15294
Mistake in AtomicBool spinlock example

The current example of a spinlock was not correct. The lock is actually acquired
when old == result. So we only need to deschedule when this is not the case.
typeck: check casts from pointers to floats, closes #15445
auto merge of #15563 : luqmana/rust/nif, r=pcwalton