Fix mutable slice syntax ...

This fixes up 100+ errors on the latest rust nightly!

See rust-lang/rust commits:
  * rust-lang/rust@3bf40568
  * rust-lang/rust@4e2afb00

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.