Add example for str replace() and MaybeOwned

  - for 3 implementations of into_maybe_owned()
  - is_slice()
  - is_owned()
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.
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.
Allow writes of length 0 to a full buffer
Add rustc --pretty flowgraph to man page
rename one of the two confusing MacroExpanders

There were two things named MacroExpander, which was confusing. I renamed
one of them TTMacroExpander.

[breaking change]
use side table to store exported macros

Per discussion with @sfackler, refactored the expander to
change the way that exported macros are collected. Specifically,
a crate now contains a side table of spans that exported macros
go into.

This has two benefits. First, the encoder doesn't need to scan through
the expanded crate in order to discover exported macros. Second, the
expander can drop all expanded macros from the crate, with the pleasant
result that a fully expanded crate contains no macro invocations (which
include macro definitions).
make walk/visit_mac opt-in only

macros can expand into arbitrary items, exprs, etc. This
means that using a default walker or folder on an AST before
macro expansion is complete will miss things (the things that
the macros expand into). As a partial fence against this, this
commit moves the default traversal of macros into a separate
procedure, and makes the default trait implementation signal
an error. This means that Folders and Visitors can traverse
macros if they want to, but they need to explicitly add an
impl that calls the walk_mac or fold_mac procedure

This should prevent problems down the road.
add Macro Exterminator

the Macro Exterminator ensures that there are no macro invocations in
an AST. This should help make later passes confident that there aren't
hidden items, methods, expressions, etc.
Update doc URLs for version bump
Bump version to 0.12.0-pre
unicode: Remove crate_id attr
rustdoc: support tuple and struct patterns in function arguments
rustdoc: render 1-tuples as (T,) instead of (T)
mark std::hash experimental

The `hash` module was not included in an earlier pass that sets baseline
stability of modules within `std` to `experimental`.