Fix wallet serialization to not use "" as a key anywhere
Fix wallet serialization to not use "" as a key anywhere

Turns out TOML does not support tables named "", so we instead encode
the accounts list as an array rather than a name-keyed hashmap. This
is fine since the account name is in the account structure itself
anyway.

apoelstra/rust-bitcoin,nacardin/rust-bitcoin,apoelstra/rust-bitcoin