This commit implements the stabilization of the custom hasher support intended
for 1.7 but left out due to some last-minute questions that needed some
decisions. A summary of the actions done in this PR are:

Stable

* `std::hash::BuildHasher`
* `BuildHasher::Hasher`
* `BuildHasher::build_hasher`
* `std::hash::BuildHasherDefault`
* `HashMap::with_hasher`
* `HashMap::with_capacity_and_hasher`
* `HashSet::with_hasher`
* `HashSet::with_capacity_and_hasher`
* `std::collections::hash_map::RandomState`
* `RandomState::new`

Deprecated

* `std::collections::hash_state`
* `std::collections::hash_state::HashState` - this trait was also moved into
  `std::hash` with a reexport here to ensure that we can have a blanket impl to
  prevent immediate breakage on nightly. Note that this is unstable in both
  location.
* `HashMap::with_hash_state` - renamed
* `HashMap::with_capacity_and_hash_state` - renamed
* `HashSet::with_hash_state` - renamed
* `HashSet::with_capacity_and_hash_state` - renamed

Closes #27713

Blocks #30884

r? @nikomatsakis

cc @durka

All structs and their constructors are defined as `DefStruct`.
`DefTy` is splitted into `DefEnum` and `DefTyAlias`.
Ad hoc flag `bool is_structure` is removed from `DefVariant`, it was required in one place in resolve and could be obtained by other means.
Flag `bool is_ctor` is removed from `DefFn`, it wasn't really used for constructors outside of metadata decoding.

Observable effects:
More specific error messages are selected in some cases.
Two name resolution bugs fixed (https://github.com/rust-lang/rust/issues/30992 and FIXME in compile-fail/empty-struct-braces-expr.rs).

Fixes https://github.com/rust-lang/rust/issues/30992
Closes https://github.com/rust-lang/rust/issues/30361

Fixes #28518.

As an attempt to make loop body destination be optional, author implemented a pretty self contained
change and deemed it to be (much) uglier than the alternative of just keeping the unit temporary.
Having the temporary created lazily also has a nice property of not figuring in the MIR of
functions which do not use loops of any sort.

r? @nikomatsakis

We've been seeing a lot of timeouts in tests on the bots and investigation ended
pointing to jemalloc/jemalloc#315 as the culprit. Unfortunately it looks like
that doesn't seem to have a fix on the way soon, so let's temporarily downgrade
back to the previous version of jemalloc we were using (where #30434 was the
most recent upgrade)

We've been seeing a lot of timeouts in tests on the bots and investigation ended
pointing to jemalloc/jemalloc#315 as the culprit. Unfortunately it looks like
that doesn't seem to have a fix on the way soon, so let's temporarily downgrade
back to the previous version of jemalloc we were using (where #30434 was the
most recent upgrade)

Tracking issue: #30014

This implements the RFC and makes a few other changes.
I have added a few extra tests, and made the Windows and
Unix code as similar as possible.

Part of the RFC mentions the unstable OpenOptionsExt trait
on Windows (see #27720). I have added a few extra methods
to future-proof it for CreateFile2.

Mostly testing borrowck and typeck behaviour in presence of `loop` expression

…ving elements into it.

Fixes #31029.

Otherwise it is not Send and Sync anymore

Minimal fix for https://github.com/rust-lang/rust/issues/30563

This covers all the public structs I think; except for Iter and
IntoIter, which I don't know if or how they should be handled.

Blocks #30884

r? @steveklabnik

An attempt to make loop body destination be optional, author implemented a pretty self contained
change and deemed it to be (much) uglier than the alternative of just keeping the unit temporary.
Having the temporary created lazily also has a nice property of not figuring in the MIR of
functions which do not use loops of any sort.

Fixes #30655

Supersedes https://github.com/rust-lang/rust/pull/30517
Fixes https://github.com/rust-lang/rust/issues/29575

cc @luqmana
r? @nikomatsakis

- Successful merges: #30981, #30982, #30986, #30987, #30988, #30990, #30998
- Failed merges:

CC #30642

r? @Gankro

I'm using clang 7.2 which works just fine to compile Rust with, but was disallowed.