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