Orchard- to Ironwood-pool Migration

Moving existing Orchard-pool funds into the Ironwood pool is an ordinary version 6 transaction rather than a special protocol operation: it spends Orchard-pool notes and creates Ironwood-pool outputs. It is the case where the post-NU6.3 rules compose. The Orchard-pool side has a positive value balance (value leaves the Orchard pool, which the value rule permits), and the Ironwood-pool side has a negative value balance (value enters the Ironwood pool); the two net to the fee.

Exact wallet migration mechanics to be described.