As is customary leave a line of white space between functions.

No need for a line of whitespace immediately starting an impl block.

Re-order the import statements in `witness` module to separate `crate`
imports from dependency imports.

This also removes tests for JSON backward-compatible encoding. Human-readable 
encoding will be changed in the next commit and this will break backward 
compatibility, thus that part of the test is removed.

Previous implementations of Witness (and Vec<Vec<u8>>) serde serialization
didn't support human-readable representations. This resulted in long unreadable
JSON/YAML byte arrays, which were especially ugly when pretty-printed (a line
per each byte).

Co-authored-by: Tobin C. Harding <me@tobin.cc>