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()
Guide: match
Guide: looping
Guide: Standard input
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.
rustc: Forbid plugin_registrar in only rlib form

If a plugin registrar is available, the library must be found in dylib form, not
just in rlib form.

Closes #15475
Document event_loop_factory usage

Add a couple of lines mentioning event_loop_factory - no clear error message is
given if you attempt to perform I/O in tasks created in this fashion.
More robust install.sh: do runnability test in fresh subdirectory.

Fix #15558.
rustc: Always mark #[crate_name] as used

It's just an annoying error if you use --crate-name on the command line and you
also have a #[crate_name] specified
rustc: Exclude #[repr(C)] from non camel case

C structs predominately do not use camel case identifiers, and we have a clear
indicator for what's a C struct now, so excuse all of them from this stylistic
lint.
Some documentation fixes and improvements
install: Make the LD_LIRARY_PATH warning more aesthetic
install: Run ldconfig when installing on Unix. Closes #15596.

If ldconfig fails it emits a warning. This is very possible when installing
to a non-system directory, so the warning tries to indicate that it may
not be a problem.
rustdoc: Add stability dashboard

This commit adds a crate-level dashboard summarizing the stability
levels of all items for all submodules of the crate.

The information is also written as a json file, intended for consumption
by pages like http://huonw.github.io/isrustfastyet/

Closes #13541
Bug fixes for stability tracking

This commit adds correct stability tracking for struct fields and
corrects some places where rustdoc was not pulling the stability data.
webfonts: name fix
rustdoc: Change type name of raw pointer from *T to *const T

Update the formatting of raw immutable pointers to print *const T.
auto merge of #15575 : mvdnes/rust/spinlock_error, r=alexcrichton

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.
auto merge of #15576 : mrmonday/rust/patch-1, r=alexcrichton

Add a couple of lines mentioning event_loop_factory - no clear error message is given if you attempt to perform I/O in tasks created in this fashion. I spent a many hours debugging this yesterday which would have been avoided if it were documented.