rustc: Disable rpath settings by default

This commit disables rustc's emission of rpath attributes into dynamic libraries
and executables by default. The functionality is still preserved, but it must
now be manually enabled via a `-C rpath` flag.

This involved a few changes to the local build system:

* --disable-rpath is now the default configure option
* Makefiles now prefer our own LD_LIBRARY_PATH over the user's LD_LIBRARY_PATH
  in order to support building rust with rust already installed.
* The compiletest program was taught to correctly pass through the aux dir as a
  component of LD_LIBRARY_PATH in more situations.

The major impact of this change is that neither rustdoc nor rustc will work
out-of-the-box in all situations because they are dynamically linked. It must be
arranged to ensure that the libraries of a rust installation are part of the
LD_LIBRARY_PATH. The default installation paths for all platforms ensure this,
but if an installation is in a nonstandard location, then configuration may be
necessary.

Additionally, for all developers of rustc, it will no longer be possible to run
$target/stageN/bin/rustc out-of-the-box. The old behavior can be regained
through the `--enable-rpath` option to the configure script.

This change brings linux/mac installations in line with windows installations
where rpath is not possible.

Closes #11747
[breaking-change]
Change DST syntax: type -> Sized?

closes #13367

[breaking-change] Use `Sized?` to indicate a dynamically sized type parameter or trait (used to be `type`). E.g.,

```
trait Tr for Sized? {}

fn foo<Sized? X: Share>(x: X) {}
```
Guide: complex data types
auto merge of #15521 : nick29581/rust/type, r=pcwalton

closes #13367

[breaking-change] Use `Sized?` to indicate a dynamically sized type parameter or trait (used to be `type`). E.g.,

```
trait Tr for Sized? {}

fn foo<Sized? X: Share>(x: X) {}
```
Guide: comments
std: Rename the `ToStr` trait to `ToString`, and `to_str` to `to_string`.

[breaking-change]
auto merge of #15493 : brson/rust/tostr, r=pcwalton

This updates https://github.com/rust-lang/rust/pull/15075.

Rename `ToStr::to_str` to `ToString::to_string`. The naive renaming ends up with two `to_string` functions defined on strings in the prelude (the other defined via `collections::str::StrAllocating`). To remedy this I removed `StrAllocating::to_string`, making all conversions from `&str` to `String` go through `Show`. This has a measurable impact on the speed of this conversion, but the sense I get from others is that it's best to go ahead and unify `to_string` and address performance for all `to_string` conversions in `core::fmt`. `String::from_str(...)` still works as a manual fast-path.

Note that the patch was done with a script, and ended up renaming a number of other `*_to_str` functions, particularly inside of rustc. All the ones I saw looked correct, and I didn't notice any additional API breakage.

Closes #15046.
auto merge of #14832 : alexcrichton/rust/no-rpath, r=brson

This commit disables rustc's emission of rpath attributes into dynamic libraries
and executables by default. The functionality is still preserved, but it must
now be manually enabled via a `-C rpath` flag.

This involved a few changes to the local build system:

* --disable-rpath is now the default configure option
* Makefiles now prefer our own LD_LIBRARY_PATH over the user's LD_LIBRARY_PATH
  in order to support building rust with rust already installed.
* The compiletest program was taught to correctly pass through the aux dir as a
  component of LD_LIBRARY_PATH in more situations.

The major impact of this change is that neither rustdoc nor rustc will work
out-of-the-box in all situations because they are dynamically linked. It must be
arranged to ensure that the libraries of a rust installation are part of the
LD_LIBRARY_PATH. The default installation paths for all platforms ensure this,
but if an installation is in a nonstandard location, then configuration may be
necessary.

Additionally, for all developers of rustc, it will no longer be possible to run
$target/stageN/bin/rustc out-of-the-box. The old behavior can be regained
through the `--enable-rpath` option to the configure script.

This change brings linux/mac installations in line with windows installations
where rpath is not possible.

Closes #11747
[breaking-change]
auto merge of #15374 : steveklabnik/rust/comments, r=brson

I'm leaving off `rustdoc` usage because it won't work unless this is a `pub fn`, and I want to talk about public/private in the context of modules. I'm also not mentioning `//!` because it is exclusively used to provide the overview of a module.
auto merge of #15422 : steveklabnik/rust/guide_compound_data_types, r=brson

I'm not happy about the hand-waving around `cmp`, but I'm not sure how to get around it.