Rename assert_eq arguments to left and right.

The names expected and actual are not used anymore in the output. It also
removes the confusion that the argument order is the opposite of junit.
Make BTree's Handle system more generic and more powerful, removing some runtine checks in favor of newly gained static safety
Emit warning when lifetime names are shadowed.

This is not technically a [breaking-change], but it will be soon, so
you should update your code. Typically, shadowing is accidental, and
the shadowing lifetime can simply be removed. This frequently occurs
in constructor patterns:

```rust
// Old:
impl<'a> SomeStruct<'a> { fn new<'a>(..) -> SomeStruct<'a> { ... } }

// Should be:
impl<'a> SomeStruct<'a> { fn new(..) -> SomeStruct<'a> { ... } }
```

Otherwise, you should rename the inner lifetime to something
else. Note though that lifetime elision frequently applies:

```rust
// Old
impl<'a> SomeStruct<'a> {
    fn get<'a>(x: &'a self) -> &'a T { &self.field }
}

// Should be:
impl<'a> SomeStruct<'a> {
    fn get(x: &self) -> &T { &self.field }
}
``
Remove all shadowed lifetimes.
Test for associated types ICE

closes #19121
auto merge of #19478 : nick29581/rust/assoc-ice-test, r=nikomatsakis

closes #19121

r?

This won't actually pass until https://github.com/rust-lang/rust/pull/19391 lands
auto merge of #19777 : nikomatsakis/rust/warn-on-shadowing, r=acrichto

per rfc 459
cc https://github.com/rust-lang/rust/issues/19390

One question is: should we start by warning, and only switch to hard error later? I think we discussed something like this in the meeting. 

r? @alexcrichton
auto merge of #19782 : gereeter/rust/cleanup-btree-node, r=Gankro

Before:
```
test btree::map::bench::find_rand_100                      ... bench:        12 ns/iter (+/- 0)
test btree::map::bench::find_rand_10_000                   ... bench:        13 ns/iter (+/- 1)
test btree::map::bench::find_seq_100                       ... bench:        11 ns/iter (+/- 0)
test btree::map::bench::find_seq_10_000                    ... bench:        11 ns/iter (+/- 1)
test btree::map::bench::insert_rand_100                    ... bench:       106 ns/iter (+/- 1)
test btree::map::bench::insert_rand_10_000                 ... bench:       326 ns/iter (+/- 8)
test btree::map::bench::insert_seq_100                     ... bench:       198 ns/iter (+/- 1)
test btree::map::bench::insert_seq_10_000                  ... bench:       312 ns/iter (+/- 3)
test btree::map::bench::iter_1000                          ... bench:     16563 ns/iter (+/- 173)
test btree::map::bench::iter_100000                        ... bench:   1686508 ns/iter (+/- 108592)
test btree::map::bench::iter_20                            ... bench:       365 ns/iter (+/- 25)
```

After:
```
test btree::map::bench::find_rand_100                      ... bench:        12 ns/iter (+/- 0)
test btree::map::bench::find_rand_10_000                   ... bench:        12 ns/iter (+/- 0)
test btree::map::bench::find_seq_100                       ... bench:        11 ns/iter (+/- 0)
test btree::map::bench::find_seq_10_000                    ... bench:        11 ns/iter (+/- 0)
test btree::map::bench::insert_rand_100                    ... bench:        89 ns/iter (+/- 1)
test btree::map::bench::insert_rand_10_000                 ... bench:       121 ns/iter (+/- 3)
test btree::map::bench::insert_seq_100                     ... bench:       149 ns/iter (+/- 0)
test btree::map::bench::insert_seq_10_000                  ... bench:       228 ns/iter (+/- 1)
test btree::map::bench::iter_1000                          ... bench:     16965 ns/iter (+/- 220)
test btree::map::bench::iter_100000                        ... bench:   1687836 ns/iter (+/- 18746)
test btree::map::bench::iter_20                            ... bench:       366 ns/iter (+/- 21)
```
Create a snapshot on top of 1b97cd3
auto merge of #19647 : nielsegberts/rust/master, r=pnkfelix

The names expected and actual are not used anymore in the output. It also
removes the confusion that the argument order is the opposite of junit.

Bug #7330 is relevant.
auto merge of #19921 : FlaPer87/rust/snapshot, r=nikomatsakis

r? @nikomatsakis