The docs for Instant::duration_since has a confusing section on panicking. It's
much more clear without the second two sentences of description.

This change will fix the issue from
rust-embedded/svd2rust#130

This commit includes the following:

* Fix syntax errors in Python 3
* Include more docstrings in classes, methods, and functions
* Include unit tests using `unittest`
* Merge implementation of `{rustc,cargo}_out_of_date`
* Merge implementation of `RustBuild.{cargo,rustc}`
* Remove unnecessary source code
* Move all the attributes defined outside of `__init__`
* Remove remaining `%s` from print function
* Remove `WindowsError` reference on non-windows systems
* Rename some variables to be more explicit avoid their meaning
* Run bootstrap tests in the CI process
* Remove non-pythonic getters
* Remove duplicate code in `download_stage0` method
* Reduce the number of branches in `build_bootstrap` method
* Re-raise exception when we cannot execute uname in non-windows systems
* Avoid long lines

rust-lang/cargo#4123 added the [patch] section of the manifest

bootstrap: Add doctests and unitests

This commit includes the following changes:

* Include more docstrings in classes, methods, and functions
* Add doctests, which are great for self-documenting our source code
* Add some unit tests with the `unittest` module
* Remove `WindowsError` reference on non-windows systems
* Rename some variables to be more explicit about their meaning
* Move all the attributes defined outside of `__init__`
* Add initial support for Python 3

r? @alexcrichton

Improve panic docs for Instant::duration_since

The docs for Instant::duration_since has a confusing section on panicking. It's
much more clear without the second two sentences of description.

Add #[derive(Clone)] to TokenTreeIter

I've found this useful for writing backtracking parsers.

The underlying `Cursor` implements `Clone` already, so it's just a matter of adding `#[derive(Clone)]` to the type.

r? @jseyfried

configure: allow distros to disable debuginfo-only-std

This allows builders to generate debugging information for everything, even in a stable release build. This is useful for distros like Fedora (already carrying a [similar patch](https://src.fedoraproject.org/cgit/rpms/rust.git/tree/rust-1.16.0-configure-no-override.patch)) and Debian that automatically put all debuginfo in separate "debug symbol" packages.

This commit preserves the default behaviour of switching these on when a non-dev channel is selected, but allows the user to override this via the `./configure` command line.

In theory, one could also do this via `bootstrap/config.toml` but it doesn't work currently due to rust-lang#43295.

[LLVM] Fix an assertion when a weak symbol is defined in global_asm.

This change will fix the issue from
rust-embedded/svd2rust#130

cc @japaric
r? @alexcrichton

Update Cargo to ffab51954ec32d55631c37a8730bb24915fc090b

rust-lang/cargo#4123 added the `[patch]` section of the manifest

Update liblibc

This updates liblibc to the latest master and fixes rust-lang#43305

Add an extra note to `late_bound_lifetime_arguments` error/lint

Fixes rust-lang#42868 (comment)