storage: add an EscapedByteSlice convenience wrapper

[hdevalence]

This may be helpful for debugging.

[hdevalence]

@zbuc I added this after seeing test output like

 insert nonconsensus change: [115, 99, 116, 47, 115, 116, 117, 98, 47, 115, 116, 97, 116, 101, 95, 99, 111, 109, 109, 105, 116, 109, 101, 110, 116, 95, 116, 114, 101, 101] -> [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 32, 0, 0, 0, 0, 0, 0, 0, 192, 180, 162, 133, 154, 38, 152, 23, 92, 175, 29, 115, 76, 223, 47, 11, 57, 3, 107, 124, 187, 99, 63, 190, 34, 149, 163, 200, 221, 245, 123, 13]

in the tests for #2429. Maybe it would be easier to read if we put this into the StateDelta as a tracing event using EscapedByteSlice at trace level or something? EDIT: did this as part of this commit.