Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
This writer is generally useful, not just for writing mutations as json. Make it generally available as well.
Soon, we will want to convert mutation fragments into json inside the scylla codebase, not just in tools. To avoid scylla-core code having to include tools/ (and link against it), move the low-level json utilities into mutation/.