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
Convert a first batch of diagnostics to have error codes
libstd: Add a few doc examples

This patch adds doc examples for the make_absolute, change_dir,
errors_string and args functions in the os module.
native: Search the child's PATH on win32

In order to have the spawning semantics be the same for unix/windows, the
child's PATH environment variable needs to be searched rather than the parent's
environment variable.

If the child is inheriting the parent's PATH, then no action need be taken as
windows will do the heavy lifting. If the child specifies its own PATH, then it
is searched beforehand for the target program and the result is favored if a hit
is found.

cc #15149, but does not close the issue because libgreen still needs to be
updated.
auto merge of #15158 : alexcrichton/rust/windows-paths, r=brson

In order to have the spawning semantics be the same for unix/windows, the
child's PATH environment variable needs to be searched rather than the parent's
environment variable.

If the child is inheriting the parent's PATH, then no action need be taken as
windows will do the heavy lifting. If the child specifies its own PATH, then it
is searched beforehand for the target program and the result is favored if a hit
is found.

cc #15149, but does not close the issue because libgreen still needs to be
updated.
mandelbrot: fix overlapping buffers
auto merge of #15649 : catharsis/rust/rust-libstd-examples, r=alexcrichton

This patch adds doc examples for the make_absolute, change_dir,
errors_string and args functions in the os module.
auto merge of #15497 : jasonthompson/rust/docs/str3, r=cmr

  - for 3 implementations of into_maybe_owned()
  - is_slice()
  - is_owned()
auto merge of #15653 : erickt/rust/master, r=alexcrichton
glob::MatchOptions struct fields should be public
Document that Result.unwrap prints the Err's value

It is implied by the Show bound, but that implication can be missed.
auto merge of #15632 : masklinn/rust/patch-1, r=alexcrichton

I saw that it was bounded by `Show` but the implication is no guarantee (and had only 0.10 to test, where this behavior has been added to 0.11)
auto merge of #15655 : lightsofapollo/rust/glob-match-options-pub, r=alexcrichton

Not sure how to test this correctly I assume the current tests pass now because of the crate boundaries [and that this is fallout from private by default]?