Parametrize LedgerTables by blk

[jasagredo]

          I'm not too happy with the `Castable` constraint, and I already have ideas for how to get rid of it. But maybe we should do it in a follow-up to this main body of UTXO-HD work.

The idea is to not parameterise LedgerTables by the ledger state type l but by the block type blk. Castable is currently capturing the fact that key/value types are the same regardless of whether l ~ LedgerState blk, or l ~ ExtLedgerState blk, or l ~ Ticked (LedgerState blk). This is true for all block types:

type instance Key   (LedgerTables l) = Key l
type instance Value (LedgerTables l) = Value l
type instance Key   (Ticked1 l)      = Key l
type instance Value (Ticked1 l)      = Value l

The reason I did not implement this change initially is that I trouble fitting this into the code, because some parts are blissfully unaware of the fact that a type parameter l, representing (ticked/extended) ledger states, always has a blk type parameter in practice. It would be a largish refactoring to make code aware of the blk parameter where necessary.

Originally posted by @jorisdral in #1267 (comment)