Implementing ByteSlice and ByteSliceMut for Vec<u8>

[shashitnak]

Adresses #992

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[joshlf]

Okay, now that #1215 and #1218 have landed, this can be rebased and the soundness issues should be addressable. In particular, we can implement ByteSlice and ByteSliceMut for Vec<u8>, but we can't implement any of the other traits.