Fix mutable slice syntax ...

This fixes up 100+ errors on the latest rust nightly!

See rust-lang/rust commits:
  * [3bf40568](https://github.com/alexcrichton/rust/commit/3bf405682df4dfb55b969640b811b317fa55447d)
  * [4e2afb00](https://github.com/alexcrichton/rust/commit/3bf405682df4dfb55b969640b811b317fa55447d)

Which require using the slicing methods in certain instances where
mutability of a slice cannot be "inferred."

I applied this fix using regular expressions; so there are likely instances where `rustc` _could've infer mutability_ for the experimental slicing syntax.

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So... **this** must be why the new slicing syntax is behind a feature flag.