Fix mutable slice syntax ...

See rust-lang/rust commits:
  * 3bf405682df4dfb55b969640b811b317fa55447d
  * 4e2afb0052618ca3d758fffd0cf50559be774391

Which requires using the slicing methods in certain instances where
mutability of a slice cannot be "inferred."

This is a mostly mechanical transform; and it may have affected code
that did not _need_ to use the explicit slicing methods.
Merge pull request #193 from drbawb/feature/fix-mut-slices

Fix mutable slice syntax ...