Avoid extra copy when unescaping JSON strings

[whitslack]

Make use of rustyrussell/ccan#123 to avoid making an extra copy of escaped JSON strings just to unescape them.

Note that jsonrpc_command_add(…) no longer accepts usage strings containing invalid escape sequences. (Previously, it would quietly accept such a string without unescaping anything.)

Checklist

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• The changelog has been updated in the relevant commit(s) according to the guidelines. Not significant enough to warrant a changelog entry.
• Tests have been added or modified to reflect the changes. Existing tests already check relevant functionality.
• Documentation has been reviewed and updated as needed. Not a user-facing change.
• Related issues have been listed and linked, including any that this PR closes. None relevant.