Improve wallet address API

[LLFourn]

get_address with an enum of AddressIndex no longer makes much sense because the Peek variant doesn't require Wallet to be mutable but the others do.

Instead I think the methods should roughly follow what's in KeychainTracker right now.

• LastUnused should be removed and replaced with a method unused_addresses which returns and iterator. If you really want the last one then you can do wallet.unused_addresses().last(). I know this is not what LastUnused does but this is what most people thinks it does. I'm not sure what
• get_address should instead just take an index and so should do what Peek currently does.
• getting a new address should be handled by a new method reveal_next_address()
• And revealing up to a certain index can be reveal_addresses_to
• The mark_used and unmark_used feature in KeychainTxOutIndex should probably be available to users of Wallet.
• The methods should all take a KeychainKind as their argument (rather than having separate methods for each).

[ValuedMammal]

I would find it more intuitive if unmark_used was called mark_unused, is that just me?