Add range lint for float literals, fixing #10934
typeck: check casts from pointers to floats, closes #15445
auto merge of #14519 : hirschenberger/rust/issue-10934, r=alexcrichton

Issue #10934
auto merge of #15559 : fhahn/rust/issue-15445-mut-cast, r=alexcrichton

I've added an error message for casts from raw pointers to floats #15445.
Stabilization for `owned` (now `boxed`) and `cell`

This PR is the outcome of the library stabilization meeting for the
`liballoc::owned` and `libcore::cell` modules.

Aside from the stability attributes, there are a few breaking changes:

* The `owned` modules is now named `boxed`, to better represent its
  contents. (`box` was unavailable, since it's a keyword.) This will
  help avoid the misconception that `Box` plays a special role wrt
  ownership.

* The `AnyOwnExt` extension trait is renamed to `BoxAny`, and its `move`
  method is renamed to `downcast`, in both cases to improve clarity.

* The recently-added `AnySendOwnExt` extension trait is removed; it was
  not being used and is unnecessary.

[breaking-change]