json: New function json_to_ds().

json: New function json_to_ds().

Some upcoming code wants to serialize JSON into a "struct ds" dynamic
string buffer, so expose an interface to do this.

This commit doesn't change much, but it renames some functions internal
to json.c to make the naming more consistent.

Also, make jsonrpc_log_msg() use this new function, since it is a more
straightforward way to do what it wants.