Readability improvements

[jcp19]

• Drop file slice_contents.gobra and the predicate AbsSlice_Any
• Drop the IO_ prefix for the types declared in the io package
• Rename predicate AbsSlice_Bytes to Bytes
• Drop wrappers (PR Simplify validity criteria of paths #352)
• Drop explicit underlying buffer parameters #358
• Simplify specs of decodeLayers #362