libs: stabilize most numerics after RFC changes

This commit adds stability markers for the APIs that have recently been
aligned with [numerics
reform](https://github.com/rust-lang/rfcs/pull/369). For APIs that were
changed as part of that reform, `#[unstable]` is used to reflect the
recency, but the APIs will become `#[stable]` in a follow-up pass.

In addition, a few aspects of the APIs not explicitly covered by the RFC
are marked here -- in particular, constants for floats.

This commit does not mark the `uint` or `int` modules as `#[stable]`,
given the ongoing debate out the names and roles of these types.

Due to some deprecation (see the RFC for details), this is a:

[breaking-change]
Fallout from deprecation
auto merge of #18999 : aturon/rust/stab-floats, r=alexcrichton,alexcrichton

This commit adds stability markers for the APIs that have recently been aligned with [numerics reform](https://github.com/rust-lang/rfcs/pull/369). For APIs that were changed as part of that reform, `#[unstable]` is used to reflect the recency, but the APIs will become `#[stable]` in a follow-up pass.

In addition, a few aspects of the APIs not explicitly covered by the RFC are marked here -- in particular, constants for floats.

This commit does not mark the `uint` or `int` modules as `#[stable]`, given the ongoing debate out the names and roles of these types.

Due to some deprecation (see the RFC for details), this is a:

[breaking-change]

r? @alexcrichton 
cc @bjz
Remove Runtime trait and bookkeeping module

This commit removes most of the remaining runtime infrastructure related
to the green/native split. In particular, it removes the `Runtime` trait
and instead inlines the native implementation.

It also changes the semantics of Rust programs so that they exit when
the main thread terminates, rather than implicitly joining on all
running tasks.

Closes #17325

[breaking-change]
Remove libnative

With runtime removal complete, there's nothing left of libnative. This
commit removes it.

Fixes #18687

[breaking-change]
Remove libgreen

With runtime removal complete, there is no longer any reason to provide
libgreen.

[breaking-change]
Rewrite sync::mutex as thin layer over native mutexes

Previously, sync::mutex had to split between green and native runtime
systems and thus could not simply use the native mutex facility.

This commit rewrites sync::mutex to link directly to native mutexes; in
the future, the two will probably be coalesced into a single
module (once librustrt is pulled into libstd wholesale).
Fallout from libgreen and libnative removal
Make most of std::rt private

Previously, the entire runtime API surface was publicly exposed, but
that is neither necessary nor desirable. This commit hides most of the
module, using librustrt directly as needed. The arrangement will need to
be revisited when rustrt is pulled into std.

[breaking-change]
Fallout from new termination semantics
Fallout from namespaced enums
Loosen possibly bogus constraints in backtrace test
Disable dubious pipe test