Forbid items with the same name from appearing in overlapping inherent impl blocks

For example, the following is now correctly illegal:

```rust
struct Foo;

impl Foo {
    fn id() {}
}

impl Foo {
    fn id() {}
}
```

"Overlapping" here is determined the same way it is for traits (and in fact shares the same code path): roughly, there must be some way of substituting any generic types to unify the impls, such that none of the `where` clauses are provably unsatisfiable under such a unification.

Along the way, this PR also introduces an `ImplHeader` abstraction (the first commit) that makes it easier to work with impls abstractly (without caring whether they are trait or inherent impl blocks); see the first commit.

Closes #22889
r? @nikomatsakis

Rollup of 11 pull requests

- Successful merges: #32137, #32158, #32171, #32174, #32178, #32179, #32180, #32181, #32183, #32186, #32197
- Failed merges:

Add FreeBSD amd64 snapshot

Snapshot itself: https://github.com/dhuseby/rust-manual-snapshots/pull/3

cleanup int suffixes in libcoretest

Clarify doc for slice slicing (Index impls)

Clarify doc for slice slicing (Index impls)

This is a follow up for PR #32099 and #32057

removed suffixes for librustc_front

removed integer suffixes in librustc_typeck crate

cover more linux targets in libstd cargobuild

libstd/build.rs checked the target name against `"unknown-linux"`... That doesn't necessarily apply to all Linux targets as some toolchains for embedded targets will change that `unknown` field to some vendor name. Some shifting around was needed since Android is also a Linux target.

r? @alexcrichton

Update getting-started.md

In `rustc 1.7.0` the message that is displayed is now `Rust is ready to roll.`

Spell fixes for std::ffi doc comments

Fix a couple compiletest nits.

lint: mark associated types as live for the dead_code pass

Associated types of trait impls were being excluded from the live list. So types that only appeared in trait impls were being marked as dead code.

Add missing documentation examples for BinaryHeap.

As part of the ongoing effort to document all methods with examples,
this commit adds the missing examples for the `BinaryHeap` collection
type.

This is part of issue #29348.

r? @steveklabnik

Add regression test for duplicate items in overlapping impls. Note that a regression test already exists for *non*overlapping impls.

impl blocks.

For example, the following is now correctly illegal:

```rust
struct Foo;

impl Foo {
    fn id() {}
}

impl Foo {
    fn id() {}
}
```

"Overlapping" here is determined the same way it is for traits (and in
fact shares the same code path): roughly, there must be some way of
substituting any generic types to unify the impls, such that none of the
`where` clauses are provably unsatisfiable under such a unification.

Closes #22889

This commit introduces the idea of an "impl header", which consists of
everything outside the impl body: the Self type, the trait
reference (when applicable), and predicates from `where` clauses. This
type is usable with the type folding machinery, making it possible to
work with impl headers at a higher and more generic level.

Snapshot itself: https://github.com/dhuseby/rust-manual-snapshots/pull/3

Forbid glob-importing from a type alias

This PR forbids glob-importing from a type alias or trait (fixes #30560):
```rust
type Alias = ();
use Alias::*; // This is currently allowed but shouldn't be
```

This is a [breaking-change]. Since the disallowed glob imports don't actually import anything, any breakage can be fixed by removing the offending glob import.

r? @alexcrichton

Add a link validator to rustbuild

This commit was originally targeted at just adding a link checking script to the rustbuild system. This ended up snowballing a bit to extend rustbuild to be amenable to various tools we have as part of the build system in general.

There's a new `src/tools` directory which has a number of scripts/programs that are purely intended to be used as part of the build system and CI of this repository. This is currently inhabited by rustbook, the error index generator, and a new linkchecker script added as part of this PR. I suspect that more tools like compiletest, tidy scripts, snapshot scripts, etc will migrate their way into this directory over time.

The commit which adds the error index generator shows the steps necessary to add new tools to the build system, namely:

1. New steps are defined for building the tool and running the tool
2. The dependencies are configured
3. The steps are implemented

In terms of the link checker, these commits do a few things:

* A new `src/tools/linkchecker` script is added. This will read an entire documentation tree looking for broken relative links (HTTP links aren't followed yet).
* A large number of broken links throughout the documentation were fixed. Many of these were just broken when viewed from core as opposed to std, but were easily fixed.
* A few rustdoc bugs here and there were fixed

rustc: Add an i586-pc-windows-msvc target

Similarly to #31629 where an i586-unknown-linux-gnu target was added, there is
sometimes a desire to compile for x86 Windows as well where SSE2 is disabled.
This commit mirrors the i586-unknown-linux-gnu target and simply adds a variant
for Windows as well.

This is motivated by a recent [Gecko bug][ff] where crashes were seen on 32-bit
Windows due to users having CPUs that don't support SSE2 instructions. It was
requested that we could have non-SSE2 builds of the standard library available
so they could continue to use vanilla releases and nightlies.

[ff]: https://bugzilla.mozilla.org/show_bug.cgi?id=1253202

Add help for E0514

I fixed #30622.

r? @arielb1

std: Add impl of FnOnce to AssertRecoverSafe

This was originally intended, but forgot to land by accident!

cc #27719

rustdoc: correct src-link url

It would have been nice for htmldocck to catch this, especially since there are rustdoc tests specifically for checking src-links.

fixes #32113

This is a follow up for PR #32099 and #32057

Never return an error after a partial write

If LineWriter fails to flush, return the number of bytes written instead
of an error.

Fixes #32085

In `rustc 1.7.0` the message that is displayed is now `Rust is ready to roll.`

Add info in `rustdoc --passes list`

CC @mitaa, https://github.com/rust-lang/rust/pull/32055/files#r55205006

Rollup of 8 pull requests

- Successful merges: #31830, #32091, #32125, #32136, #32147, #32148, #32149, #32150
- Failed merges:

Remove inaccurate claim about inline assembly

It's not like GCC's.

Fixes #20213

Add other primitive types to the reference

Fixes #31628

Small grammar fix in Guessing Game

When it was Option.expect(), there was an .ok().expect(), but now that it uses Result.expect(), there's only one method, not two.

Fixes #31912

Clarify that try_unwrap needs exactly one

Fixes #31950

Add missing documentation examples for BTreeSet.

As part of the ongoing effort to document all methods with examples,
this commit adds the missing examples for the `BTreeSet` collection
type.

This is part of issue #29348.

r? @steveklabnik