Deprecate as_mut_slice methods

[erickt]

This is technically a breaking change as it deprecates and unstables some previously stable apis that were missed in the last round of deprecations.

[breaking change]

[rust-highfive]

r? @pcwalton

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[alexcrichton]

@bors: r+ 5ed703b

Thanks!

[bors]

⌛ Testing commit 5ed703b with merge ffcc8c4...

[bors]

💔 Test failed - auto-mac-32-opt

[tamird]

failures:

---- distributions::WeightedChoice_0 stdout ----
    <anon>:12:40: 12:54 error: use of unstable library feature 'collections': use &mut s[..] instead
<anon>:12     let wc = WeightedChoice::new(items.as_mut_slice());

[erickt]

@bors: retry

[erickt]

@bors: r+ rollup c5b8763

[bors]

📌 Commit c5b8763 has been approved by erickt